Peer manager cfg (#2766)

## Issue Addressed
I've done this change in a couple of WIPs already so I might as well submit it on its own. This changes no functionality but reduces coupling in a 0.0001%. It also helps new people who need to work in the peer manager to better understand what it actually needs from the outside

## Proposed Changes

Add a config to the peer manager

beacon_node/lighthouse_network/src/lib.rs

@@ -11,7 +11,7 @@ mod config;
 #[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 pub mod discovery;
 mod metrics;
-mod peer_manager;
+pub mod peer_manager;
 pub mod rpc;
 mod service;
 pub mod types;