Configure the validator/register_validator batch size via the CLI (#4399)

## Issue Addressed

NA

## Proposed Changes

Adds the `--validator-registration-batch-size` flag to the VC to allow runtime configuration of the number of validators POSTed to the [`validator/register_validator`](https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Validator/registerValidator) endpoint.

There are builders (Agnostic and Eden) that are timing out with `regsiterValidator` requests with ~400 validators, even with a 9 second timeout. Exposing the batch size will help tune batch sizes to (hopefully) avoid this.

This PR should not change the behavior of Lighthouse when the new flag is not provided (i.e., the same default value is used).

## Additional Info

NA

validator_client/src/cli.rs

@@ -333,6 +333,16 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .default_value("true")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("validator-registration-batch-size")
+                .long("validator-registration-batch-size")
+                .value_name("INTEGER")
+                .help("Defines the number of validators per \
+                    validator/register_validator request sent to the BN. This value \
+                    can be reduced to avoid timeouts from builders.")
+                .default_value("500")
+                .takes_value(true),
+        )
         /*
          * Experimental/development options.
          */