Define general purpose and optimized read-only API for Galley

[ayj]

The configuration component has two logical sets of APIs: the general purpose RESTful API and the more specialized read-only sync API for high throughput configuration consumers. Both are gRPC APIs.

The RESTful version of the general purpose API is provided by grpc-gateway and later possibly through envoy grpc transcoding. An overview of this method is documented here.

ref https://docs.google.com/document/d/13UqmYpEjNGT2xucIgWaTQIQUgQuAASeIdm8Ks3zp5Fk/edit#bookmark=id.1ttx3vf6ypj2

[ayj]

cc @mandarjog

[geeknoid]

No longer relevant.