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.

lighthouse/tests/validator_client.rs

@@ -129,6 +129,22 @@ fn use_long_timeouts_flag() {
         .with_config(|config| assert!(config.use_long_timeouts));
 }
 
+#[test]
+fn long_timeouts_multiplier_flag_default() {
+    CommandLineTest::new()
+        .run()
+        .with_config(|config| assert_eq!(config.long_timeouts_multiplier, 1));
+}
+
+#[test]
+fn long_timeouts_multiplier_flag() {
+    CommandLineTest::new()
+        .flag("use-long-timeouts", None)
+        .flag("long-timeouts-multiplier", Some("10"))
+        .run()
+        .with_config(|config| assert_eq!(config.long_timeouts_multiplier, 10));
+}
+
 #[test]
 fn beacon_nodes_tls_certs_flag() {
     let dir = TempDir::new().expect("Unable to create temporary directory");