BeaconLC code re-use between nimbus_verified_proxy and portal bridge

[kdeme]

There is definitely a lot of code duplication between the fluffy beacon_chain_bridge.nim, nimbus_verified_proxy.nim and the original beacon light client in nimbus-eth2.

nimbus_verified_proxy did a lot of copying to be able to get a PoC up fast, but it has shown that maintaining it (e.g. each fork there are some updates to do typically) is a bit of a pain. Check if/how this can be improved.