Support Cross Namespace PoolObjectReferences

[danehans]

Since InferencePool and InferenceModel resources are managed by different personas they may reside in different namespaces. Therefore, PoolObjectReference should support an optional namespace field that defines the namespace of the referenced object. When this field is unspecified or specifies the same namespace as the InferencePool, the referenced object must be in the same namespace as the InferencePool. When namespace specifies a different namespace from the InferencePool, a ReferenceGrant object is required in the referenced object's namespace to allow the reference.

[enjoyliu]

I want to try it

[ahg-g]

I discussed this with @robscott briefly in the initial API proposal and we opted to start without allowing cross namespace referencing of resources to avoid dealing with that complexity and not to have dependencies on the upstream gateway apis initially. @robscott wdyt?

With ReferenceGrant, is it the extension itself that needs to honor the policy?