Add RPC support over socket connection

[britzl]

RPCs over the Socket to complement existing RPC functionality of Client (See https://heroiclabs.com/docs/nakama/client-libraries/unity/index.html#socket-rpcs)

[novabyte]

@britzl I think this task should be to add socket RPCs as well as keeping the existing client RPCs functionality. We usually recommend that client RPCs are used wherever possible.

[britzl]

@novabyte Thanks. I copied the description from the email conversation earlier with @tomglenn. Changing description to not read "instead of Client".

[msesen]

I need to manually disconnect a socket session connection. Been able to call an RPC on the socket object will expose the session_id in the context variable, which then is a matter of calling the nk.session_disconnect() on the server.

This feature is available in other clients, so would be very useful to have this for Defold as well.