webrtc: janus: handle slowlink event

Fix this warning:

webrtc-janusvr-signaller imp.rs:426:gstrswebrtc::janusvr_signaller:👿:Signaller::handle_msg:<GstJanusVRWebRTCSignallerU64@0x7f317009b4d0> Unknown message from server: {
   "janus": "slowlink",
   "session_id": 980554280060589,
   "sender": 5867141593320621,
   "mid": "video0",
   "media": "video",
   "uplink": false,
   "lost": 15
}

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1929>

net/webrtc/src/janusvr_signaller/imp.rs

@@ -185,6 +185,17 @@ struct InnerHangup {
     reason: String,
 }
 
+#[derive(Serialize, Deserialize, Debug)]
+struct InnerSlowLink {
+    session_id: u64,
+    sender: u64,
+    opaque_id: Option<String>,
+    mid: String,
+    media: String,
+    uplink: bool,
+    lost: u64,
+}
+
 #[derive(Serialize, Deserialize, Debug)]
 struct RoomJoined {
     room: JanusId,
@@ -267,6 +278,7 @@ enum JsonReply {
     Media,
     Error(InnerError),
     HangUp(InnerHangup),
+    SlowLink(InnerSlowLink),
 }
 
 #[derive(Default)]
@@ -561,7 +573,7 @@ impl Signaller {
             }
             JsonReply::HangUp(hangup) => self.raise_error(format!("hangup: {}", hangup.reason)),
             // ignore for now
-            JsonReply::Ack | JsonReply::Media => {}
+            JsonReply::Ack | JsonReply::Media | JsonReply::SlowLink(_) => {}
         }
     }