This changes two aspects of the ResourceProvider.Update RPC API:
1. Update needs to return an ID, in case the resource had to be
recreated in response to the request.
2. Include both the old and the new values for properties that are
being updated.
After a bit more thinking, we will create new SDK packages for each
of the languages we wish to support writing resource providers in.
This is where the RPC goo will live, so I have created a new sdk/
directory, moved the Protobuf/gRPC definitions underneath sdk/proto/,
and put the generated code into sdk/go/ and sdk/js/.