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.

book/src/help_vc.md

@@ -251,6 +251,13 @@ Flags:
           contain sensitive information about your validator and so this flag
           should be used with caution. For Windows users, the log file
           permissions will be inherited from the parent folder.
+      --long-timeouts-multiplier <LONG_TIMEOUTS_MULTIPLIER>
+          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. [default: 1]
       --metrics
           Enable the Prometheus metrics HTTP server. Disabled by default.
       --prefer-builder-proposals