Support Replace mergeType field in BTP and SP

[arkodg]

@zhaohuabing good point, we can rm the CEL and support it for BTP and SP in a follow up

Originally posted by @arkodg in #8169 (comment)

[illrill]

+1, given that docs indicate mergeType: Replace is supported on SecurityPolicy

Is this just to remove the CEL validations and logically handle it same way as if it's unset?

[mehara-rothila]

Hi, I'd like to work on this if it's still available. Could a maintainer please assign it to me?

[mehara-rothila]

Hi @zhaohuabing, thanks for assigning this. It's ready for review: it removes the CEL block on mergeType: Replace for BackendTrafficPolicy and SecurityPolicy, implements the Replace merge in internal/utils/merge.go (the more specific policy fully replaces the parent), and regenerates the CRDs, with tests and docs. As a first-time contributor my workflows are gated, could you approve the CI run when you get a chance? Happy to address any feedback. Thanks!

[illrill]

Small but impactful. As a cluster operator I have a Gateway-level "default" SP. I want to allow application developer to define Route-level SP, but I want to deny (via Kyverno) their SP if doesn't set mergeType explicitly to either StrategicMerge or Replace (force application developer to make a concious decision). Now it's not so easy to achieve this