Pin it to BeaconBlock. The methods and structs are really specific to beacon
setup, so there's nearly no use to generalize this.

This adopts the full networking stack from lighthouse

Code is adopted from lighthouse, which is licensed under Apache-2. Unfortunately
because the dependencies we use are different, it's not possible to directly
depend on upstream.

So that we're on a version that supports gossipsub, thanks to lighthouse.

This copies blockchain-network-libp2p crate as the foundation to continue to
build upon.