* op-node,op-service: Add Fallback Beacon Client

This splits out a very thin wrapper around the Beacon API into the
BeaconClient interface. This interface is then implement by two different
structs. The first is a simple wrapper around the HTTP Client. The second
is a FallBack Client which tries two different underlying clients.

This also a beacon archiver to be used as a fallback while still pulling most
of the relevant data from the beacon node.

* Try all clients in the pool