Health Endpoints for UI (#3668)

This PR adds some health endpoints for the beacon node and the validator client.

Specifically it adds the endpoint:
`/lighthouse/ui/health`

These are not entirely stable yet. But provide a base for modification for our UI. 

These also may have issues with various platforms and may need modification.

beacon_node/lighthouse_network/src/service/behaviour.rs

@@ -8,7 +8,6 @@ use libp2p::gossipsub::subscription_filter::{
 };
 use libp2p::gossipsub::Gossipsub as BaseGossipsub;
 use libp2p::identify::Identify;
-use libp2p::swarm::NetworkBehaviour;
 use libp2p::NetworkBehaviour;
 use types::EthSpec;