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/config.rs

@@ -77,6 +77,8 @@ pub struct Config {
     pub disable_run_on_all: bool,
     /// Enables a service which attempts to measure latency between the VC and BNs.
     pub enable_latency_measurement_service: bool,
+    /// Defines the number of validators per `validator/register_validator` request sent to the BN.
+    pub validator_registration_batch_size: usize,
 }
 
 impl Default for Config {
@@ -117,6 +119,7 @@ impl Default for Config {
             gas_limit: None,
             disable_run_on_all: false,
             enable_latency_measurement_service: true,
+            validator_registration_batch_size: 500,
         }
     }
 }
@@ -380,6 +383,12 @@ impl Config {
         config.enable_latency_measurement_service =
             parse_optional(cli_args, "latency-measurement-service")?.unwrap_or(true);
 
+        config.validator_registration_batch_size =
+            parse_required(cli_args, "validator-registration-batch-size")?;
+        if config.validator_registration_batch_size == 0 {
+            return Err("validator-registration-batch-size cannot be 0".to_string());
+        }
+
         /*
          * Experimental
          */