Update friendly validator client

### 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_