feat(swarm): add #[non_exhaustive] to key enums

Add `#[non_exhaustive]` to the following enums so that future additions don't cause breaking changes in downstream client code: `FromSwarm`, `ToSwarm`, `SwarmEvent`, `ConnectionHandlerEvent`, `ConnectionEvent`.

Related: libp2p#4543.

Pull-Request: libp2p#4581.

src/behaviour.rs

@@ -593,6 +593,7 @@ impl NetworkBehaviour for Behaviour {
                 self.inner.on_swarm_event(listener_closed)
             }
             confirmed @ FromSwarm::ExternalAddrConfirmed(_) => self.inner.on_swarm_event(confirmed),
+            _ => {}
         }
     }