Use Server-Side Apply patch to manage finalizers

[enarha]

Originally using Merge Patch + resourceVersion to manage finalizers leads to many retries when multiple controllers are managing the same type of object. The issue is especially apparent for PipelineRuns and TaskRuns. Using Server-Side Apply (SSA) patch eliminates that problem as well as other issues where one controller overrides another controller field.
When using SSA patch it's mandatory to provide fieldManager indicating which controller/client is managing a particular field. To communicate that from a controller, we leverage the Knative controller AgentName option. So far none of the Tekton controllers is using that, so we can use it as an indicator of intention to use an SSA patch to manage the finalizer. When a controller does not set AgentName, we keep the original behavior. This allows seamless transition for all controllers at their own time.

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/kind feature

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[waveywaves]

Will have to close this PR for now as this change needs to be in the knative package first and then once it's updated there, we will be able to use that feature in tekton.
@enarha could you create an issue and link it here in the meanwhile ? I will close the issue after.

[waveywaves]

I had added the above comment from my phone so didn't end up seeing the PR mentioned in the issue @enarha my bad

[waveywaves]

/hold