MultiplayerSpawner:
- When spawn_path is invalid.
- When the auto spawn list is empty and _spawn_custom is not overridden.
Note: We remove the warning for placeholder scripts since there's no
way of knowing if they have a certain method.
MultiplayerSynchronizer:
- When root_path is invalid.
- 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.
