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/src/lib.rs

@@ -478,6 +478,7 @@ impl<E: EthSpec> ProductionValidatorClient<E> {
             context: duties_context,
             enable_high_validator_count_metrics: config.enable_high_validator_count_metrics,
             distributed: config.distributed,
+            disable_attesting: config.disable_attesting,
         });
 
         // Update the metrics server.
@@ -507,6 +508,7 @@ impl<E: EthSpec> ProductionValidatorClient<E> {
             .validator_store(validator_store.clone())
             .beacon_nodes(beacon_nodes.clone())
             .runtime_context(context.service_context("attestation".into()))
+            .disable(config.disable_attesting)
             .build()?;
 
         let preparation_service = PreparationServiceBuilder::new()