WIP Set podTemplate on Tasks to enable multi-arch builds with Matrix

[dorzel]

Changes

Hello! @jeffdyoung and I are working towards implementing the feature requested in: #6742, namely enabling Tekton to do builds on clusters with nodes of multiple architectures by enabling podTemplate to be set on Tasks.

This currently is an extremely basic approach without any of the supporting tests, validation, and other related code changes in order to show a proof of concept and get some feedback on the general approach before creating a TEP. This will of course later also be cleaned up to follow https://github.com/tektoncd/pipeline/blob/main/CONTRIBUTING.md.

These minimal changes do currently work to accomplish the goal, with an example Pipeline like:

kind: PipelineRun
metadata:
  generateName: matrixed-pipelinerun-
spec:
  pipelineSpec:
    tasks:
      - name: build-and-push-manifest
        matrix:
          params:
          - name: arch
            value: 
              - "amd64"
              - "arm64"
        taskSpec:
          results:
            - name: manifest
              type: string
          params:
            - name: arch
          podTemplate:
            nodeSelector:
              kubernetes.io/arch: $(params.arch)
          steps:
            - name: build-and-push
              image: ubuntu
              script: |
                echo "building on $(params.arch)"
                echo "testmanifest-$(params.arch)" | tee $(results.manifest.path)
      - name: create-manifest-list
        params:
          - name: manifest
            value: $(tasks.build-and-push-manifest.results.manifest[*])
        taskSpec:
          steps:
            - name: echo-manifests
              image: ubuntu
              args: ["$(params.manifest[*])"]
              script: echo "$@"

Feedback appreciated, thanks!

Fixes #6742

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
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Release Notes

NONE

[linux-foundation-easycla]

• ❌ - login: @dorzel / name: Dylan Orzel . The commit (81ca27d, a9b05dc) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

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[afrittoli]

/kind feature

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/pod.go	93.3%	92.9%	-0.3
pkg/reconciler/taskrun/resources/apply.go	99.4%	98.7%	-0.6

[jeffdyoung]

cc: @aleskandro @Prashanth684 @arewm @adambkaplan