Add --long-timeouts-multiplier CLI flag (#7047)

Adds the `--long-timeouts-multiplier` flag.
Allows granular control for VC timeouts which has proved useful in Holesky.

validator_client/src/cli.rs

@@ -116,6 +116,20 @@ pub struct ValidatorClient {
     )]
     pub use_long_timeouts: bool,
 
+    #[clap(
+        long,
+        requires = "use_long_timeouts",
+        default_value_t = 1,
+        help = "If present, the validator client will use a multiplier for the timeout \
+                when making requests to the beacon node. This only takes effect when \
+                the `--use-long-timeouts` flag is present. The timeouts will be the slot \
+                duration multiplied by this value. This flag is generally not recommended, \
+                longer timeouts can cause missed duties when fallbacks are used.",
+        display_order = 0,
+        help_heading = FLAG_HEADER,
+    )]
+    pub long_timeouts_multiplier: u32,
+
     #[clap(
         long,
         value_name = "CERTIFICATE-FILES",