Revive mplex (#4619)

## Issue Addressed

N/A

## Proposed Changes

In #4431 , we seem to have removed support for mplex as it is being deprecated in libp2p. See libp2p/specs#553 . Related rust-libp2p PR libp2p/rust-libp2p#3920
However, since this isn't part of the official [consensus specs](https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/p2p-interface.md#multiplexing), we still need to support mplex. 

> Clients MUST support [mplex](https://github.com/libp2p/specs/tree/master/mplex) and MAY support [yamux](https://github.com/hashicorp/yamux/blob/master/spec.md).

This PR adds back mplex support as before.

beacon_node/lighthouse_network/Cargo.toml

@@ -44,6 +44,7 @@ prometheus-client = "0.21.0"
 unused_port = { path = "../../common/unused_port" }
 delay_map = "0.3.0"
 void = "1"
+libp2p-mplex = "0.40.0"
 
 [dependencies.libp2p]
 version = "0.52"

beacon_node/lighthouse_network/src/service/utils.rs

@@ -50,13 +50,21 @@ pub fn build_transport(
         transport.or_transport(libp2p::websocket::WsConfig::new(trans_clone))
     };
 
+    // mplex config
+    let mut mplex_config = libp2p_mplex::MplexConfig::new();
+    mplex_config.set_max_buffer_size(256);
+    mplex_config.set_max_buffer_behaviour(libp2p_mplex::MaxBufferBehaviour::Block);
+
     // yamux config
     let mut yamux_config = yamux::Config::default();
     yamux_config.set_window_update_mode(yamux::WindowUpdateMode::on_read());
     let (transport, bandwidth) = transport
         .upgrade(core::upgrade::Version::V1)
         .authenticate(generate_noise_config(&local_private_key))
-        .multiplex(yamux_config)
+        .multiplex(core::upgrade::SelectUpgrade::new(
+            yamux_config,
+            mplex_config,
+        ))
         .timeout(Duration::from_secs(10))
         .boxed()
         .with_bandwidth_logging();