refactor: expose and use THandlerOutEvent type alias (#3368)

Previously, we used the full reference to the `OutEvent` of the `ConnectionHandler` in all implementations of `NetworkBehaviour`. Not only is this very verbose, it is also more brittle to changes. With the current implementation plan for #2824, we will be removing the `IntoConnectionHandler` abstraction. Using a type-alias to refer to the `OutEvent` makes the migration much easier.

protocols/autonat/src/behaviour.rs

@@ -37,8 +37,8 @@ use libp2p_swarm::{
         AddressChange, ConnectionClosed, ConnectionEstablished, DialFailure, ExpiredExternalAddr,
         ExpiredListenAddr, FromSwarm,
     },
-    ConnectionHandler, ConnectionId, ExternalAddresses, IntoConnectionHandler, ListenAddresses,
-    NetworkBehaviour, NetworkBehaviourAction, PollParameters,
+    ConnectionId, ExternalAddresses, ListenAddresses, NetworkBehaviour, NetworkBehaviourAction,
+    PollParameters, THandlerOutEvent,
 };
 use std::{
     collections::{HashMap, VecDeque},
@@ -553,8 +553,7 @@ impl NetworkBehaviour for Behaviour {
         &mut self,
         peer_id: PeerId,
         connection_id: ConnectionId,
-        event:  <<Self::ConnectionHandler as IntoConnectionHandler>::Handler as
-    ConnectionHandler>::OutEvent,
+        event: THandlerOutEvent<Self>,
     ) {
         self.inner
             .on_connection_handler_event(peer_id, connection_id, event)