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 https://github.com/libp2p/specs/issues/553 . Related rust-libp2p PR https://github.com/libp2p/rust-libp2p/pull/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"