feat: add cloud workflows quickstart (GoogleCloudPlatform#5200)

## Description

Adds Cloud Workflows quickstart.

Tested on Cloud Shell:

```
GOOGLE_CLOUD_PROJECT=my-project python3 main.py
```

## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
  - workflows.googleapis.com
  - workflowexecutions.googleapis.com
- [x] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [x] Please **merge** this PR for me once it is approved.
- [x] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

---

Test:

```
cd ~/github/googlecloudplatform/python-docs-samples/
python3 -m venv my-venv
source my-venv/bin/activate 
cd workflows/cloud-client
pip3 install -r requirements-test.txt 
cd ..
GOOGLE_CLOUD_PROJECT=serverless-com-demo nox -s py-3.7 -- cloud-client/main_test.py
```

Local execution:

```
GOOGLE_CLOUD_PROJECT=serverless-com-demo python3 cloud-client/main.py     
```

Author: grant

.github/CODEOWNERS

@@ -68,3 +68,4 @@
 /translate/**/*.py                     @telpirion @sirtorry @GoogleCloudPlatform/python-samples-owners
 /video/**/*.py                         @telpirion @sirtorry @GoogleCloudPlatform/python-samples-owners
 /vision/**/*.py                        @telpirion @sirtorry @GoogleCloudPlatform/python-samples-owners
+/workflows/**/*.py                      @grant @GoogleCloudPlatform/python-samples-owners

workflows/cloud-client/README.md

@@ -0,0 +1,52 @@
+<img src="https://avatars2.githubusercontent.com/u/2810941?v=3&s=96" alt="Google Cloud Platform logo" title="Google Cloud Platform" align="right" height="96" width="96"/>
+
+# Cloud Workflows Quickstart – Python
+
+This sample shows how to execute a Cloud Workflow and wait for the workflow execution results using the Python client libraries.
+
+## Setup
+
+1. Deploy the workflow, `myFirstWorkflow`:
+
+    1. Copy the YAML from this file: https://github.com/GoogleCloudPlatform/workflows-samples/blob/main/src/myFirstWorkflow.workflows.yaml
+    1. Paste the YAML into a file called `myFirstWorkflow.workflows.yaml`.
+    1. Run the command: `gcloud workflows deploy myFirstWorkflow --source myFirstWorkflow.workflows.yaml`
+
+## Run the Quickstart
+
+Install [`pip`][pip] and [`virtualenv`][virtualenv] if you do not already have them.
+
+You may want to refer to the [`Python Development Environment Setup Guide`][setup] for Google Cloud Platform for instructions.   
+
+1. Create a virtualenv. Samples are compatible with Python 2.7 and 3.4+.
+
+    ```sh
+    virtualenv env
+    source env/bin/activate
+    ```
+
+1. Install the dependencies needed to run the samples.
+
+    ```sh
+    pip install -r requirements.txt
+    ```
+
+1. Start the application, setting your project name in an environment variable, `GOOGLE_CLOUD_PROJECT`:
+
+    ```sh
+    export GOOGLE_CLOUD_PROJECT=your-project-id
+    python main.py
+    ```
+
+1. Observe the results:
+
+    In stdout, you should see a JSON response from your workflow like the following:
+
+    ```json
+    ["Wednesday","Wednesday Night Wars","Wednesday 13","Wednesday Addams","Wednesday Campanella","Wednesdayite","Wednesday Martin","Wednesday Campanella discography","Wednesday Night Hockey (American TV program)","Wednesday Morning, 3 A.M."]
+    ```
+
+[prereq]: ../README.md#prerequisities
+[setup]: https://cloud.google.com/python/setup
+[pip]: https://pip.pypa.io/
+[virtualenv]: https://virtualenv.pypa.io/

workflows/cloud-client/main.py

@@ -0,0 +1,70 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+
+def execute_workflow(
+    project, location="us-central1", workflow="myFirstWorkflow"
+):
+    """Execute a workflow and print the execution results."""
+    # [START workflows_api_quickstart]
+    import time
+
+    from google.cloud import workflows_v1beta
+    from google.cloud.workflows import executions_v1beta
+    from google.cloud.workflows.executions_v1beta.types import executions
+
+    # TODO(developer): Uncomment these lines and replace with your values.
+    # project = 'my-project-id'
+    # location = 'us- central1'
+    # workflow = 'myFirstWorkflow'
+
+    if not project:
+        raise Exception('GOOGLE_CLOUD_PROJECT env var is required.')
+
+    # Set up API clients.
+    execution_client = executions_v1beta.ExecutionsClient()
+    workflows_client = workflows_v1beta.WorkflowsClient()
+
+    # Construct the fully qualified location path.
+    parent = workflows_client.workflow_path(project, location, workflow)
+
+    # Execute the workflow.
+    response = execution_client.create_execution(request={"parent": parent})
+    print(f"Created execution: {response.name}")
+
+    # Wait for execution to finish, then print results.
+    execution_finished = False
+    backoff_delay = 1  # Start wait with delay of 1 second
+    print('Poll every second for result...')
+    while (not execution_finished):
+        execution = execution_client.get_execution(request={"name": response.name})
+        execution_finished = execution.state != executions.Execution.State.ACTIVE
+
+        # If we haven't seen the result yet, wait a second.
+        if not execution_finished:
+            print('- Waiting for results...')
+            time.sleep(backoff_delay)
+            backoff_delay *= 2  # Double the delay to provide exponential backoff.
+        else:
+            print(f'Execution finished with state: {execution.state.name}')
+            print(execution.result)
+            return execution.result
+    # [END workflows_api_quickstart]
+
+
+if __name__ == "__main__":
+    project = os.environ.get('GOOGLE_CLOUD_PROJECT')
+    execute_workflow(project=project)

workflows/cloud-client/main_test.py

@@ -0,0 +1,58 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+from google.cloud import workflows_v1beta
+
+import main
+
+PROJECT = os.environ["GOOGLE_CLOUD_PROJECT"]
+LOCATION = "us-central1"
+WORKFLOW_ID = "myFirstWorkflow"
+
+
+def test_workflow_execution():
+    assert PROJECT != ""
+
+    if not workflow_exists():
+        workflow_file = open("myFirstWorkflow.workflows.yaml", "r").read()
+
+        workflows_client = workflows_v1beta.WorkflowsClient()
+        workflows_client.create_workflow(request={
+            # Manually construct the location
+            # https://github.com/googleapis/python-workflows/issues/21
+            "parent": f'projects/{PROJECT}/locations/{LOCATION}',
+            "workflow_id": WORKFLOW_ID,
+            "workflow": {
+                "name": WORKFLOW_ID,
+                "source_contents": workflow_file
+            }
+        })
+
+    result = main.execute_workflow(PROJECT)
+    assert len(result) > 0
+
+
+def workflow_exists():
+    """Returns True if the workflow exists in this project
+    """
+    try:
+        workflows_client = workflows_v1beta.WorkflowsClient()
+        workflow_name = workflows_client.workflow_path(PROJECT, LOCATION, WORKFLOW_ID)
+        workflows_client.get_workflow(request={"name": workflow_name})
+        return True
+    except Exception as e:
+        print(f"Workflow doesn't exist: {e}")
+        return False

workflows/cloud-client/myFirstWorkflow.workflows.yaml

@@ -0,0 +1,29 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+- getCurrentTime:
+    call: http.get
+    args:
+      url: https://us-central1-workflowsample.cloudfunctions.net/datetime
+    result: currentTime
+- readWikipedia:
+    call: http.get
+    args:
+      url: https://en.wikipedia.org/w/api.php
+      query:
+        action: opensearch
+        search: ${currentTime.body.dayOfTheWeek}
+    result: wikiResult
+- returnResult:
+    return: ${wikiResult.body[1]}

workflows/cloud-client/requirements-test.txt

@@ -0,0 +1 @@
+pytest==6.2.1

workflows/cloud-client/requirements.txt

@@ -0,0 +1 @@
+google-cloud-workflows==0.2.0