Registry should support a `?format` parameter similar to the gateway

[criccomini]

The schema registry HTTP/JSON endpoint only returns RecapTypes (as dicts) right now. It should allow users to request the schemas in JSON schema, Protobuf, or Avro as well, similar to the gateway.

The storage layer should continue to just return RecapTypes, though. It's lower level, and I don't want to put conversion logic in there.