Support Advanced Templating for KMS credentials

[Xkynar]

With External Secrets Operator you can transform the data from the external secret provider before it is stored as Secret: https://external-secrets.io/main/guides/templating/

Currently ack-secret-manager only supports jmesPath to grab keys from a JSON-formatted secret, but this is very limited. Below are two ESO examples that I don't think are possible with this solution:

# Simple: Add a prefix to the secret
apiVersion: external-secrets.io/v1
kind: ExternalSecret
metadata:
  name: dynatrace-token
spec:
  secretStoreRef:
    name: my-secret-store
    kind: SecretStore
  data:
    - secretKey: apiToken
      remoteRef:
        key: dynatrace-access-secret
  target:
    name: dynatrace-token
    template:
      data:
        dynatraceToken: "Api-Token {{ `{{ .apiToken }}` }}"

# Advanced: Create a config yaml from the secret
apiVersion: external-secrets.io/v1
kind: ExternalSecret
metadata:
  name: thanos-objstore-secret
spec:
  secretStoreRef:
    name: my-secret-store
    kind: SecretStore
  data:
    - secretKey: access_key_id
      remoteRef:
          key: thanos-access-secret
          property: username
    - secretKey: access_key_secret
      remoteRef:
          key: thanos-access-secret
          property: password
  target:
    name: thanos-objstore-secret
    creationPolicy: Owner
    template:
      data:
        objstore.yml: |
          type: s3
          config:
            bucket: mybucket
            endpoint: s3.eu-central-1.amazonaws.com
            access_key_id: "{{ `{{ .access_key_id }}` }}"
            access_key_secret: "{{ `{{ .access_key_secret }}` }}"

Templating is very useful since a secret may have to be formatted differently for different apps according to their expectations. Storing the same secret multiple times pre-formatted in KMS is possible but not the cleanest solution.