Separate Protocol class into CoreProtocol, ServerProtocol, and ClientProtocol

[hlbarber]

At the time of writing, there is a single Protocol class which holds both client and server methods, despite it living inside the codegen folder. The codegen folder is supposed to be server agnostic.

We should separate Protocol interface into CoreProtocol, ServerProtocol, and ClientProtocol interfaces.

• CoreProtocol holds methods common to both the server and client.
• ServerProtocol holds an instance of CoreProtocol and lives in the codegen-server.
• ClientProtocol should work similarly to ServerProtocol - encapsulating all client specific methods and living in an appropriate space.

As a result, the classes implementing Protocol should also be separated. For example:

• class CoreRestJsonProtocol: CoreProtocol
• class ServerRestJsonProtocol: ServerProtocol
• class ClientRestJsonProtocol: ClientProtocol

and methods should be added to Server{Protocol}Protocol and Client{Protocol}Protocol to retrieve Core{Protocol}Protocol.

[hlbarber]

Related progress towards this goal: #1697

[hlbarber]

The ServerProtocol class has been introduced in #1693. Further work needs to be done to move existing methods from the implementations of Protocol onto the implementations of ServerProtocol.

[hlbarber]

and methods should be added to Server{Protocol}Protocol and Client{Protocol}Protocol to retrieve Core{Protocol}Protocol.

Rather than have this be the case, it's probably more ergonomic to have ServerProtocol : CoreProtocol and Server{Protocol}Protocol : Core{Protocol}Protocol, ServerProtocol. Similarly, ClientProtocol: CoreProtocol and Client{Protocol}Protocol: Core{Protocol}Protocol, ClientProtocol.

Changes have been made in #1693 to do this for the server side.