✨Implement opaque declarative validation marker

[itzPranshul]

What does this do, and why do we need it?

When a Go type has type-level validation markers, those markers are automatically inherited by all fields that use that type. This PR introduces +k8s: opaque, a field-level marker that suppresses type-level validation inheritance for a specific field.

This marker gives users a way to opt-out of type level validations when reusing a go type in a CRD struct field, so they can define custom field level validations without inheriting any conflicts.

Which issue(s) this PR is related to:

Fixes #1301

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @itzPranshul. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[alvaroaleman]

/ok-to-test

Can you please also add an envtest-based test?