Add `disableAttesting` flag to vc

### Problem description

Inspired by Lighthouse, to disable vc attesting via a flag.

The original intent of this flag is to reduce burden on CL and EL when they are struggling to sync, and subsequently remove the flag to start attesting upon synced.

However this feature is also deemed useful during Holesky fallout where we want to keep proposing blocks but not attesting. Currently we rely on slashing protection db which do not stop us completely from attesting but only those attestations that would cause slashing.

### Solution description

Add `disableAttesting` flag to vc

### Additional context

_No response_