Add --disable-attesting flag to validator client (#7046)

Cleaned up and isolated version of the `--disable-attesting` flag for the VC, from the `holesky-rescue` branch:

- https://github.com/sigp/lighthouse/pull/7041

I figured we don't need the `--disable-attesting` flag on the BN for now, and it was a much more invasive impl.

book/src/help_vc.md

@@ -175,6 +175,10 @@ Flags:
           If this flag is set, Lighthouse will query the Beacon Node for only
           block headers during proposals and will sign over headers. Useful for
           outsourcing execution payload construction during proposals.
+      --disable-attesting
+          Disable the performance of attestation duties (and sync committee
+          duties). This flag should only be used in emergencies to prioritise
+          block proposal duties.
       --disable-auto-discover
           If present, do not attempt to discover new validators in the
           validators-dir. Validators will need to be manually added to the