[Net] Single rpc annotation. "sync" no longer part of mode.

- Move the "sync" property for RPCs to RPCConfig.

- Unify GDScript annotations into a single one:
  - `@rpc(master)` # default
  - `@rpc(puppet)`
  - `@rpc(any)` # former `@remote`

- Implement three additional `@rpc` options:
  - The second parameter is the "sync" option (which also calls the
    function locally when RPCing). One of "sync", "nosync".
  - The third parameter is the transfer mode (reliable, unreliable,
    ordered).
  - The third parameter is the channel (unused for now).

modules/gdscript/gdscript.h

@@ -64,7 +64,6 @@ class GDScript : public Script {
 		int index = 0;
 		StringName setter;
 		StringName getter;
-		MultiplayerAPI::RPCMode rpc_mode;
 		GDScriptDataType data_type;
 	};