Change Stream.provider from string to ObjectRef

[ericbottard]

No description provided.

[scothis]

Let's hold off on this one. There are significant complexity and usability concerns introduced by using an ObjectReference, that I don't think are worth the benefit have have different providers with the same name.

1. tools (or users) creating a stream need to know about the specific provisions types API Group and kind (like streaming.projectriff.io/v1alpha1 and KafkaProvider)
2. the controller reconciling the stream needs RBAC permissions, duck types and a rest client to be able to get/watch the provider type

[ericbottard]

Closing in favor of #86 to keep loose coupling