[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/doc_classes/SceneMultiplayer.xml

@@ -42,6 +42,10 @@
 			The root path to use for RPCs and replication. Instead of an absolute path, a relative path will be used to find the node upon which the RPC should be executed.
 			This effectively allows to have different branches of the scene tree to be managed by different MultiplayerAPI, allowing for example to run both client and server in the same scene.
 		</member>
+		<member name="server_relay" type="bool" setter="set_server_relay_enabled" getter="is_server_relay_enabled" default="true">
+			Enable or disable the server feature that notifies clients of other peers' connection/disconnection, and relays messages between them. When this option is [code]false[/code], clients won't be automatically notified of other peers and won't be able to send them packets through the server.
+			[b]Note:[/b] Support for this feature may depend on the current [MultiplayerPeer] configuration. See [method MultiplayerPeer.is_server_relay_supported].
+		</member>
 	</members>
 	<signals>
 		<signal name="peer_packet">