Feature: Support beaconcha.in stats collection

[yorickdowne]

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 and Teku.

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