Feature: Support beaconcha.in stats collection
**Is your feature request related to a problem? Please describe.**
Nimbus cannot natively send stats to beaconcha.in, and the non-native method does not send validator information. An Eth Docker user inquired about seeing Nimbus stats on their beaconcha.in mobile app.

**Describe the solution you'd like**
Implement metrics push to beaconcha.in natively. The spec is here: https://github.com/gobitfly/eth2-client-metrics

Clients that currently support this collection natively are Lighthouse, Lodestar, Teku and Grandine. Prysm has it in a separate executable.

**Describe alternatives you've considered**
Teach the eth2-client-metrics-exporter how to get validator metrics from Nimbus
