Modularize or Simplify the eth2 crate

[AgeManning]

Description

The common/eth2 crate appears to import the whole world. Other crates that depend on eth2 are compiling and building libp2p and a whole host of other things.

It would be great if we could significantly reduce the dependency chain here. Either by splitting up some of the structs or removing unnecessary imports.

[dapplion]

I remember asking myself this when doing some PoC a long time ago that depended on common/eth2

[dapplion]

The dependency on libp2p is only for these types

use lighthouse_network::{types::SyncState, PeerInfo};
use lighthouse_network::{ConnectionDirection, Enr, Multiaddr, PeerConnectionStatus};
use lighthouse_network::PeerId;