Proposal: Registry v2 authorization service

Outlines usage for endpoints which will be used by the
docker registry client for registry v2 non-standalone.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Should mention the public key being looked up by `kid`.  I would expect that the public key is registered with the registry, which may create an internal look up table from `kid` to public key.  Also should there be any mechanism which verifies the relationship between the `iss`  and `kid`?