spike initial stream abstraction

[markfisher]

This spike will define a Stream CRD that together with a Subscription CRD allows for any Kubernetes Service to be configured as a subscriber of that stream. Optionally a Broker CRD may be configured to refer to a backing implementation for streams, such as Kafka. See the document and repo links below for more detail.

Doc: https://docs.google.com/document/d/19R5twA0cd7PYGpcMDWFBiFzV3-RBMd2qfj2WzVzb8OA/edit

GitHub repo: https://github.com/scothis/stream-spike

/kind dev
/kind doc
/assign @markfisher
/assign @scothis

[scothis]

done with knative/eventing@df9d5d9

/close