Add marker to skip/ignore specific fields on CRD generation

[luisdavim]

Similar request to #521 and https://groups.google.com/g/kubebuilder/c/N2A8FYNuWLA (both ended up unanswered 😞 )

In some cases we might re-use structs for CRDs and some other configuration and want to prevent certain fields form being added to the CRDs, something like a // +skip or // +ignore would be nice.

[sbueringer]

In general I'm not sure if it's a good idea to reuse structs when there's not a 100% fit. E.g. see the explanation here: https://github.com/kubernetes/kubernetes/blob/v1.32.0/staging/src/k8s.io/api/core/v1/types.go#L6886-L6896

@JoelSpeed What do you think about supporting this?

[JoelSpeed]

In some cases we might re-use structs for CRDs and some other configuration

Can you provide a more detailed explanation of a use case for this?

Generally, from an API reviewers perspective, you want the types to be clean and focused directly on the API. Think about clients (people building tools against your API) using your types, if they pull in the Go type, how would they understand that the extra fields you have +skip on, are actually not valid fields within the API? Their IDE isn't likely to point that out to them is it?

[luisdavim]

In our case we have a packaging system and a congregation file to allow building packages parts of that configuration can be used with a controller to deploy the packages, not all the fields in the struct are relevant to the controller parts. We do use separate structs and composition to avoid this as much as possible but it's not always possible. The extra fields don't cause issues as they are ignored by the controller but it can be confusing to the user seeing them when checking the API.