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.

validator_client/validator_services/src/duties_service.rs

@@ -230,6 +230,7 @@ pub struct DutiesService<T, E: EthSpec> {
     pub enable_high_validator_count_metrics: bool,
     /// If this validator is running in distributed mode.
     pub distributed: bool,
+    pub disable_attesting: bool,
 }
 
 impl<T: SlotClock + 'static, E: EthSpec> DutiesService<T, E> {
@@ -403,6 +404,11 @@ pub fn start_update_service<T: SlotClock + 'static, E: EthSpec>(
         "duties_service_proposers",
     );
 
+    // Skip starting attestation duties or sync committee services.
+    if core_duties_service.disable_attesting {
+        return;
+    }
+
     /*
      * Spawn the task which keeps track of local attestation duties.
      */