[Net] Basic extensible MultiplayerAPI spawn/despawn.

`PackedScene`s can be configured to be spawnable via a new
`MultiplayerAPI.spawnable_config` method.
They can be configured either to be spawned automatically when coming
from the server or to always require verification.
Another method, `MultiplayerAPI.send_spawn` lets you request a spawn on
the remote peers.
When a peer receive a spawn request:
- If it comes from the server and the scene is configured as
  `SPAWN_MODE_SERVER`:
  - Spawn the scene (instantiate it, add it to tree).
  - Emit signal `network_spawn`.
- Else:
  - Emit signal `network_spawn_request`.

In a similar way, `despawn`s are handled automatically in
`SPAWN_MODE_SERVER`.

In `SPAWN_MODE_SERVER`, when a new client connects it will also receive,
from the server all the spawned (and not yet despawned) instances.

scene/main/node.cpp

@@ -116,6 +116,9 @@ void Node::_notification(int p_notification) {
 				memdelete(data.path_cache);
 				data.path_cache = nullptr;
 			}
+			if (data.filename.length()) {
+				get_multiplayer()->scene_enter_exit_notify(data.filename, this, false);
+			}
 		} break;
 		case NOTIFICATION_PATH_CHANGED: {
 			if (data.path_cache) {
@@ -147,6 +150,10 @@ void Node::_notification(int p_notification) {
 
 				get_script_instance()->call(SceneStringNames::get_singleton()->_ready);
 			}
+			if (data.filename.length()) {
+				ERR_FAIL_COND(!is_inside_tree());
+				get_multiplayer()->scene_enter_exit_notify(data.filename, this, true);
+			}
 
 		} break;
 		case NOTIFICATION_POSTINITIALIZE: {