KEP-2876: CRD Validation Expression Language (kubernetes#2877)

* WIP

* WIP

* add alternatives

* Add WebAssembly alternative section.

* Update the KEP for validation

* update title to focus on validation, expand on alternatives

* Replace TODOs with UNRESOLVED and flesh out risks

* Update KEP

* Update KEP based on feedback received.

* Address SIG bi-weekly feedback

* Add PRR review.
Update status to be implementable.

* Remove security review.

* Apply feedback

* Fill out PRR questionare

* Clarify semantic equality

* Address unsolved section.

* Move type field into future work

* Fix toc checking.

* Address need for test for rollback

* Add graduation criteria for beta

* Add test plan and graduation criteria

* Address liggitts feedback

* Update keps/sig-api-machinery/2876-crd-validation-expression-language/README.md

Co-authored-by: David Eads <deads2k@users.noreply.github.com>

* Add comments about CEL expression complexity

Co-authored-by: Kermit Alexander <kermitalexandr@google.com>
Co-authored-by: cici37 <cicih@google.com>
Co-authored-by: David Eads <deads2k@users.noreply.github.com>

keps/prod-readiness/sig-api-machinery/2876.yaml

@@ -0,0 +1,3 @@
+kep-number: 2558
+alpha:
+  approver: "@deads2k"