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/cli.rs

@@ -97,6 +97,15 @@ pub struct ValidatorClient {
     )]
     pub disable_auto_discover: bool,
 
+    #[clap(
+        long,
+        help = "Disable the performance of attestation duties (and sync committee duties). This \
+                flag should only be used in emergencies to prioritise block proposal duties.",
+        display_order = 0,
+        help_heading = FLAG_HEADER
+    )]
+    pub disable_attesting: bool,
+
     #[clap(
         long,
         help = "If present, the validator client will use longer timeouts for requests \