Update friendly validator client

[dapplion]

Problem description

Doing maintenance on a beacon node or validator client can result in brief downtime. Large operators are recommended to include fallback URLs, but a validator still experiences 12 seconds of downtime until fallback kicks in

Solution description

We could add two new routes in a validator client API:

• Route to check if any registered validator has block proposer duties: /proposer_duties
• Route to call fallback first, switch URL priority for a limited period of time: /fallback_first?duration_sec=1800

Additional context

No response