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.