Configurable monitoring endpoint frequency (#3530)

## Issue Addressed

Closes #3514

## Proposed Changes

- Change default monitoring endpoint frequency to 120 seconds to fit with 30k requests/month limit.
- Allow configuration of the monitoring endpoint frequency using `--monitoring-endpoint-frequency N` where `N` is a value in seconds.

lighthouse/tests/validator_client.rs

@@ -443,3 +443,16 @@ fn no_strict_fee_recipient_flag() {
         .run()
         .with_config(|config| assert!(!config.strict_fee_recipient));
 }
+
+#[test]
+fn monitoring_endpoint() {
+    CommandLineTest::new()
+        .flag("monitoring-endpoint", Some("http://example:8000"))
+        .flag("monitoring-endpoint-period", Some("30"))
+        .run()
+        .with_config(|config| {
+            let api_conf = config.monitoring_api.as_ref().unwrap();
+            assert_eq!(api_conf.monitoring_endpoint.as_str(), "http://example:8000");
+            assert_eq!(api_conf.update_period_secs, Some(30));
+        });
+}