Tracking: Support Nimbus Unified

[yorickdowne]

nimbus-unified.yml with REST, without the baked-in VC. Offer during ./ethd config when on testnet, and query for a VC when running a validating node

Tracking for feature support

• Max peers for both CL and EL components, Separate --max-peers in unified client status-im/nimbus-eth1#4387
• Custom network support for CL and EL components, Custom networks in unified client status-im/nimbus-eth1#4388
• Grafana dashboard without hard-coded Prometheus UID, Do not hardcode Prometheus uid in Grafana dashboard status-im/nimbus-eth1#4393