feat(swarm): Make executor for connection tasks explicit (#3097)

Previously, the executor for connection tasks silently defaulted to a `futures::executor::ThreadPool`. This causes issues such as https://github.com/libp2p/rust-libp2p/issues/2230.

With this patch, we force the user to choose, which executor they want to run the connection tasks on which results in overall simpler API with less footguns.

Closes #3068.

examples/ipfs-kad.rs

@@ -68,7 +68,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
             behaviour.add_address(&PeerId::from_str(peer)?, bootaddr.clone());
         }
 
-        Swarm::new(transport, behaviour, local_peer_id)
+        Swarm::with_async_std_executor(transport, behaviour, local_peer_id)
     };
 
     // Order Kademlia to search for a peer.