Add ClusterTask to v1alpha2 🎋

[vdemeester]

Changes

This port ClusterTask spec to v1alpha2, using v1alpha2.TaskSpec too.

/cc @sbwsg @bobcatfish

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
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Reviewer Notes

If API changes are included, additive changes must be approved by at least two OWNERS and backwards incompatible changes must be approved by more than 50% of the OWNERS, and they must first be added in a backwards compatible way.

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha2/cluster_task_defaults.go	Do not exist	0.0%
pkg/apis/pipeline/v1alpha2/cluster_task_types.go	Do not exist	0.0%
pkg/apis/pipeline/v1alpha2/cluster_task_validation.go	Do not exist	0.0%

[ghost]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg

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• OWNERS [sbwsg]

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[vdemeester]

/test pull-tekton-pipeline-integration-tests

[chmouel]

/lgtm

Looking Good! 🤙🏽