WebRTC refactor. Data channels, STUN/TURN support.

A big refactor to the WebRTC module. API is now considered quite stable.

Highlights:

- Renamed `WebRTCPeer` to `WebRTCPeerConnection`.
- `WebRTCPeerConnection` no longer act as `PacketPeer`, it only handle the connection itself (a bit like `TCP_Server`)
- Added new `WebRTCDataChannel` class which inherits from `PacketPeer` to handle data transfer.
- Add `WebRTCPeerConnection.initialize` method to create a new connection with the desired configuration provided as dictionary ([see MDN docs](https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/RTCPeerConnection#RTCConfiguration_dictionary)).
- Add `WebRTCPeerConnection.create_data_channel` method to create a data channel for the given connection. The connection must be in `STATE_NEW` as specified by the standard ([see MDN docs for options](https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/createDataChannel#RTCDataChannelInit_dictionary)).
- Add a `data_channel_received` signal to `WebRTCPeerConnection` for in-band (not negotiated) channels.
- Renamed `WebRTCPeerConnection` `offer_created` signal to `session_description_created`.
- Renamed `WebRTCPeerConnection` `new_ice_candidate` signal to `ice_candidate_created`

modules/gdnative/gdnative_api.json

@@ -6436,11 +6436,26 @@
         "next": null,
         "api": [
           {
-            "name": "godot_net_bind_webrtc_peer",
+            "name": "godot_net_set_webrtc_library",
+            "return_type": "godot_error",
+            "arguments": [
+              ["const godot_net_webrtc_library *", "p_library"]
+            ]
+          },
+          {
+            "name": "godot_net_bind_webrtc_peer_connection",
             "return_type": "void",
             "arguments": [
               ["godot_object *", "p_obj"],
-              ["const godot_net_webrtc_peer *", "p_interface"]
+              ["const godot_net_webrtc_peer_connection *", "p_interface"]
+            ]
+          },
+          {
+            "name": "godot_net_bind_webrtc_data_channel",
+            "return_type": "void",
+            "arguments": [
+              ["godot_object *", "p_obj"],
+              ["const godot_net_webrtc_data_channel *", "p_interface"]
             ]
           }
         ]