Add network feature to eth2 (#8558)

This reverts some of the changes from #8524 by adding back the typed network endpoints with an optional `network` feature. Without the `network` feature, these endpoints (and associated dependencies) will not be built.

This means the `enr`, `multiaddr` and `libp2p-identity` dependencies have returned but are now optional


Co-Authored-By: Mac L <mjladson@pm.me>

beacon_node/http_api/src/lib.rs

@@ -2140,12 +2140,9 @@ pub fn serve<T: BeaconChainTypes>(
                     let discovery_addresses = enr.multiaddr_p2p_udp();
                     Ok(api_types::GenericResponse::from(api_types::IdentityData {
                         peer_id: network_globals.local_peer_id().to_base58(),
-                        enr: enr.to_base64(),
-                        p2p_addresses: p2p_addresses.iter().map(|a| a.to_string()).collect(),
-                        discovery_addresses: discovery_addresses
-                            .iter()
-                            .map(|a| a.to_string())
-                            .collect(),
+                        enr,
+                        p2p_addresses,
+                        discovery_addresses,
                         metadata: utils::from_meta_data::<T::EthSpec>(
                             &network_globals.local_metadata,
                             &chain.spec,