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.

Cargo.lock

@@ -4117,6 +4117,25 @@ dependencies = [
  "prometheus-client",
 ]
 
+[[package]]
+name = "libp2p-mplex"
+version = "0.40.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93959ed08b6caf9810e067655e25f1362098797fef7c44d3103e63dcb6f0fabe"
+dependencies = [
+ "asynchronous-codec",
+ "bytes",
+ "futures",
+ "libp2p-core",
+ "libp2p-identity",
+ "log",
+ "nohash-hasher",
+ "parking_lot 0.12.1",
+ "rand 0.8.5",
+ "smallvec 1.11.0",
+ "unsigned-varint 0.7.1",
+]
+
 [[package]]
 name = "libp2p-noise"
 version = "0.43.0"
@@ -4382,6 +4401,7 @@ dependencies = [
  "hex",
  "lazy_static",
  "libp2p",
+ "libp2p-mplex",
  "lighthouse_metrics",
  "lighthouse_version",
  "lru 0.7.8",