feat: [cel] add CEL-based assertions and offline VAP evaluation

[crandles]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds a cel package for writing CEL-based assertions against Kubernetes objects in tests. Same expression language the API server uses for CRD validation and admission policies, so tests don't drift from the policies they exercise.

Design doc: docs/cel-assertions.md.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Happy to split by sub-package if that's easier to review -- cel, policy, feature, wait, decoder each stand alone.

Does this PR introduce a user-facing change?

Added a `cel` package for writing CEL-based assertions against Kubernetes objects in tests, including offline ValidatingAdmissionPolicy evaluation.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @crandles. Thanks for your PR.

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[k8s-triage-robot]

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[vladimirvivien]

/ok-to-test

[crandles]

Thanks! I'll dig into the failures

[vladimirvivien]

hi @crandles - this is an awesome submission (as were all of your contributions). Because of its size, it will take me some time to digest everything.

Quick feed back: consider making cel a top root package instead of klient/cel.

[crandles]

Happy to move it, naming is hard.

Open to scoping down, or splitting over multiple PRs. It seemed like CEL would be nice throughout the project.

---

On another note:

Should I consider joining the org?

[crandles]

golangci-lint version mismatch, seems I'm ahead and getting different results locally.

[crandles]

I will be proposing this here, shortly, but in the meantime: I've added a Github Workflow that runs on forked-repository pull requests. It would be nice to catch simple errors sooner.

Work in progress: crandles#1

[vladimirvivien]

@crandles yes, you should definitely consider joining the project as you've landed some great contributions in the past. We had golangci-lint version issues a few weeks back and were forced to update, so you may have to update your env.

[vladimirvivien]

@crandles Let's split the PR into functional packages as you suggested. cel and policy first (including design docs, tests, examples, etc). Then another PR for the remainder packages..

[crandles]

splitting per #562 (comment) -- #569 has the cel + cel/policy core (with an offline example). a follow-up PR will layer on cel/decoder, cel/feature, cel/wait + the live-cluster example.

closing this in favor of the split.