YAML service?

[timburks]

I wonder if we should add a service or methods to directly support Registry YAML. For modularity, I would suggest making this a separate service, perhaps like this:

service YAML {
  rpc Apply(ApplyRequestMessage) ApplyResponseMessage;
  rpc Get(GetRequestMessage) GetResponseMessage;
}

where the request and response messages correspond to the inputs and outputs of the registry apply and registry get -o yaml commands.

This could be prototyped here in the server directory.