CEL client side validation

[clux]

What problem are you trying to solve?

I want to verify that a local object satisfies a CEL validation rule as specified via Rule or #[cel_validation] attributes (via #1649 ) without having to kubectl apply / Api::patch the object and wait for apiserver validation to pass/fail.

Describe the solution you'd like

An optional feature dependency kube-core/cel and maybe kube-derive/cel (enabled via kube/cel) which adds a dependency on (e.g.) cel-interpreter to run client-side CEL evaluation against local objects.

AFAIKT, we can encode our objects as a cel Context and run the x-kubernetes-validations.rule against it. Their Value type seems to be convertible into from a serde::Serialize impl.

Describe alternatives you've considered

None, open to alternatives.

Documentation, Adoption, Migration Strategy

Addition under feature for CELSchema users. Needs documentation as with the rest of cel.

Target crate for feature

kube-core, kube-derive