whipclient: emit shutdown after DELETE request

A shutdown signal to the base object i.e. webrtcsink will
make sure the client is closed gracefully, especially when
the ICE connection with the WHIP server peer is lost in an
unexpected way

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2358>

net/webrtc/src/whip_signaller/imp.rs

@@ -428,6 +428,7 @@ impl WhipClient {
                 }
             },
         };
+        self.obj().emit_by_name::<()>("shutdown", &[])
     }
 }