Beacon Node `peer_id` in metrics

[KaloyanTanev]

🎯 Problem to be solved

We are aware that some nodes may be running the same beacon node for multiple Charon clusters. One way to capture that is the peer id of the Beacon node.

🛠️ Proposed solution

In Charon, under beaconNodeVersionMetric add an extra call to GET /eth/v1/node/identity and use the peer_id of the response to populate a new metric beaconNodePeerIDGauge

Include this as an extra column in the Beacon Node Version panel in Grafana.