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/chat.rs

@@ -115,7 +115,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
         };
 
         behaviour.floodsub.subscribe(floodsub_topic.clone());
-        Swarm::new(transport, behaviour, local_peer_id)
+        Swarm::with_threadpool_executor(transport, behaviour, local_peer_id)
     };
 
     // Reach out to another node if specified