add execution-timeout-multiplier flag to optionally increase timeouts (#3631)

## Issue Addressed
Add flag to lengthen execution layer timeouts

Which issue # does this PR address?

#3607 

## Proposed Changes

Added execution-timeout-multiplier flag and a cli test to ensure the execution layer config has the multiplier set correctly.

Please list or describe the changes introduced by this PR.
Add execution_timeout_multiplier to the execution layer config as Option<u32> and pass the u32 to HttpJsonRpc.

## Additional Info
Not certain that this is the best way to implement it so I'd appreciate any feedback.

Please provide any additional information. For example, future considerations
or information useful for reviewers.

beacon_node/src/cli.rs

@@ -503,7 +503,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .requires("execution-endpoint")
                 .takes_value(true)
         )
-
+        .arg(
+            Arg::with_name("execution-timeout-multiplier")
+                .long("execution-timeout-multiplier")
+                .value_name("NUM")
+                .help("Unsigned integer to multiply the default execution timeouts by.")
+                .default_value("1")
+                .takes_value(true)
+        )
         /*
          * Database purging and compaction.
          */