webrtc: janus: handle 'destroyed' messages from Janus

Fix this error when the room is destroyed:

ERROR   webrtc-janusvr-signaller imp.rs:413:gstrswebrtc::janusvr_signaller:👿:Signaller::handle_msg:<GstJanusVRWebRTCSignallerU64@0x55b166a3fe40> Unknown message from server: {
   "janus": "event",
   "session_id": 6667171862739941,
   "sender": 1964690595468240,
   "plugindata": {
      "plugin": "janus.plugin.videoroom",
      "data": {
         "videoroom": "destroyed",
         "room": 8320333573294267
      }
   }
}

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1481>

net/webrtc/src/janusvr_signaller/imp.rs

@@ -64,6 +64,15 @@ impl JanusId {
     }
 }
 
+impl std::fmt::Display for JanusId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            JanusId::Str(s) => write!(f, "{s}"),
+            JanusId::Num(n) => write!(f, "{n}"),
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, PartialEq, Eq)]
 struct KeepAliveMsg {
     janus: String,
@@ -179,6 +188,11 @@ struct RoomEvent {
     error: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug)]
+struct RoomDestroyed {
+    room: JanusId,
+}
+
 #[derive(Serialize, Deserialize, Debug)]
 struct RoomTalking {
     room: JanusId,
@@ -192,6 +206,7 @@ struct RoomTalking {
 enum VideoRoomData {
     Joined(RoomJoined),
     Event(RoomEvent),
+    Destroyed(RoomDestroyed),
     Talking(RoomTalking),
     StoppedTalking(RoomTalking),
 }
@@ -484,6 +499,14 @@ impl Signaller {
                                 }
                             }
                         }
+                        VideoRoomData::Destroyed(room_destroyed) => {
+                            gst::trace!(CAT, imp: self, "Room {} has been destroyed", room_destroyed.room);
+
+                            self.raise_error(format!(
+                                "room {} has been destroyed",
+                                room_destroyed.room
+                            ));
+                        }
                         VideoRoomData::Talking(talking) => {
                             self.emit_talking(true, talking.id, talking.audio_level);
                         }