POC: Concurrency prototype with separate controller

[lbernick]

Changes

This commit adds a proof of concept for a controller that reconciles ConcurrencyControls, in order to control execution of concurrent PipelineRuns.

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[tekton-robot]

Skipping CI for Draft Pull Request.
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[tekton-robot]

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[lbernick]

I've updated this POC to include an admission webhook that patches all PipelineRuns as pending on creation, and to have the reconciler start them on the first reconcile loop.

[tekton-robot]

@lbernick: PR needs rebase.

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[lbernick]

Closing in favor of #905