Request to add admission webhook for PipelineRun CRD

Why we need it

https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#admission-webhooks

We can define two types of admission webhooks, validating admission webhook and mutating admission webhook. Mutating admission webhooks are invoked first, and can modify objects sent to the API server to enforce custom defaults. After all object modifications are complete, and after the incoming object is validated by the API server, validating admission webhooks are invoked and can reject requests to enforce custom policies.

So mutating admission webhook can do set some default values for our PipelineRun CR, like owner references and some labels. And validating admission webhook can do validate some fields of PipelineRun CR.

/kind feature

Start by reading the linked Kubernetes admission webhooks documentation and locating the PipelineRun CRD and its API-server integration in this repository. Clarify which default values and validation rules are required, then identify the relevant tests or add a test plan that demonstrates both mutating and validating behavior.