Publish subscriptions to all beacon nodes (#3529)

## Issue Addressed

Resolves #3516 

## Proposed Changes

Adds a beacon fallback function for running a beacon node http query on all available fallbacks instead of returning on a first successful result. Uses the new `run_on_all` method for attestation and sync committee subscriptions. 

## Additional Info

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

lighthouse/tests/validator_client.rs

@@ -442,3 +442,19 @@ fn monitoring_endpoint() {
             assert_eq!(api_conf.update_period_secs, Some(30));
         });
 }
+#[test]
+fn disable_run_on_all_default() {
+    CommandLineTest::new().run().with_config(|config| {
+        assert!(!config.disable_run_on_all);
+    });
+}
+
+#[test]
+fn disable_run_on_all() {
+    CommandLineTest::new()
+        .flag("disable-run-on-all", None)
+        .run()
+        .with_config(|config| {
+            assert!(config.disable_run_on_all);
+        });
+}