✨ feat: Add resource field-scoped fields

[TheSpiritXIII]

This feature adds resource field-scoped fields.

Why?

While working on Google Managed Service for Prometheus, we identified a need to have namespaced and cluster-scoped versions of the same structs.

How

This solution adds a new marker on fields that appear conditionally depending on the top-level CRD scope.

For example, we may have a struct:

type SecretSelector struct {
	// These fields appear regardless of scope.
	Name string  `json:"name"`
	Key string  `json:"key"`

	// This field only appears for cluster-scoped objects, since we could use the namespace of the top-level object.
	// +kubebuilder:field:scope=Cluster
	Namespace string `json:"namespace"`
}

For a namespaced CRD, the namespace field will not be generated as part of the OpenAPI specification because the namespace field is marked as being cluster-scoped.

The opposite is true as well. A field can be marked namespaced and will not be generated for cluster-scoped resources.

Defaults apply too. A CRD is considered namespaced by default, so cluster-scoped fields will not appear.

Implementation

The implementation is admittedly a bit hacky. A special property is inserted at schema parse-time, and later removed when processing the resource. I couldn't figure out a better way to do this. The comments in the code should describe why it was done this way.