Add Support for managedBy field in TaskRun and PipelineRun

[khrm]

Feature request

We should add a "managedBy" field to delegate responsibility of controlling the lifecycle of PipelineRuns/TaskRuns.

The semantics of the field:

• whenever the value is set, and it does not point to the built-in operator, then skip reconciliation
• the field is immutable
• the field is not defaulted

Without this, at present, we have to skip installation of pipelines controller to make external controller work on PipelineRun/TaskRun/

Usecase

Allowing external schedulers like Kueue to manage the lifecycle of TaskRuns and PipelineRuns. This is particularly useful in a multi-cluster environment where a central scheduling component (like MultiKueue) needs to control which workloads run on which cluster, and when. By setting the managedBy field to a custom value, users can prevent the default Tekton controller from acting on these resources, giving full control to the external scheduler.

For context, the efforts to support the field in:

kubernetes/enhancements#4368 (beta)
kubernetes-sigs/jobset#440 (complete)

[khrm]

/assign @khrm