[TEP-0089] Enable SPIRE for signing taskrun results in alpha.

[jagathprakash]

Breaking down PR #4759 originally proposed by @pxp928 to address TEP-0089 according @lumjjb suggestions. Plan for breaking down PR is PR 1.1: api PR 1.2: entrypointer (+cmd line + test/entrypointer) Entrypoint takes results and signs the results (termination message). PR 1.3: reconciler + pod + cmd/controller + integration tests Controller will verify the signed result. This commit corresponds to 1.3 above.

Changes

Submitter Checklist

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

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• Has Tests included if any functionality added or changed
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  functionality, content, code)
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Release Notes

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[tekton-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[jagathprakash]

/test all

[jagathprakash]

/assign lumjjb

[tekton-robot]

@jagathprakash: GitHub didn't allow me to assign the following users: lumjjb.

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In response to this:

/assign lumjjb

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[jagathprakash]

/assign pxp928

[tekton-robot]

@jagathprakash: GitHub didn't allow me to assign the following users: pxp928.

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In response to this:

/assign pxp928

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[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/pod/pod.go	89.5%	90.4%	0.9
pkg/pod/status.go	90.9%	91.0%	0.1
pkg/reconciler/taskrun/controller.go	95.0%	95.5%	0.5
pkg/reconciler/taskrun/resources/image_exporter.go	81.8%	83.3%	1.5
pkg/reconciler/taskrun/taskrun.go	81.3%	79.5%	-1.8

[tekton-robot]

@jagathprakash: PR needs rebase.

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[jagathprakash]

/test pull-tekton-pipeline-build-tests

[tekton-robot]

@jagathprakash: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-tekton-pipeline-integration-tests	db10074	link	true	/test pull-tekton-pipeline-integration-tests
pull-tekton-pipeline-alpha-integration-tests	db10074	link	true	/test pull-tekton-pipeline-alpha-integration-tests
pull-tekton-pipeline-build-tests	7b290c2	link	true	/test pull-tekton-pipeline-build-tests

Full PR test history. Your PR dashboard.

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[tekton-robot]

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