[Net] Modularize multiplayer, expose MultiplayerAPI to extensions.

- RPC configurations are now dictionaries.
- Script.get_rpc_methods renamed to Script.get_rpc_config.
- Node.rpc[_id] and Callable.rpc now return an Error.
- Refactor MultiplayerAPI to allow extension.
- New MultiplayerAPI.rpc method with Array argument (for scripts).
- Move the default MultiplayerAPI implementation to a module.

core/SCsub

@@ -188,7 +188,6 @@ SConscript("os/SCsub")
 SConscript("math/SCsub")
 SConscript("crypto/SCsub")
 SConscript("io/SCsub")
-SConscript("multiplayer/SCsub")
 SConscript("debugger/SCsub")
 SConscript("input/SCsub")
 SConscript("variant/SCsub")