[MP] Let MultiplayerAPI handle packet relaying and peer signaling.

MultiplayerPeer changes:

- Adds is_server_relay_supported virtual method

Informs the upper MultiplayerAPI layer if it can signal peers connected
to the server to other clients, and perform packet relaying among them.

- Adds get_packet_channel and get_packet_mode virtual methods

Allows the MultiplayerAPI to retrieve the channel and transfer modes to
use when relaying the last received packet.

SceneMultiplayerPeer changes:

- Implement peer signaling and packet relaying when the MultiplayerPeer
  advertise they are supported.

ENet, WebRTC, WebSocket changes:

- Removed custom code for relaying from WebSocket and ENet, and let it
  be handled by the upper layer.
- Update WebRTC to split create_client, create_server, and create_mesh,
  with the latter behaving like the old initialize with
  "server_compatibility = false", and the first two supporting the upper
  layer relaying protocol.

modules/multiplayer/scene_replication_interface.cpp

@@ -370,10 +370,9 @@ Error SceneReplicationInterface::_send_raw(const uint8_t *p_buffer, int p_size,
 #endif
 
 	Ref<MultiplayerPeer> peer = multiplayer->get_multiplayer_peer();
-	peer->set_target_peer(p_peer);
 	peer->set_transfer_channel(0);
 	peer->set_transfer_mode(p_reliable ? MultiplayerPeer::TRANSFER_MODE_RELIABLE : MultiplayerPeer::TRANSFER_MODE_UNRELIABLE);
-	return peer->put_packet(p_buffer, p_size);
+	return multiplayer->send_command(p_peer, p_buffer, p_size);
 }
 
 Error SceneReplicationInterface::_make_spawn_packet(Node *p_node, MultiplayerSpawner *p_spawner, int &r_len) {