Set user agent on requests to builder (#4199)

## Issue Addressed

Closes #4185

## Proposed Changes

- Set user agent to `Lighthouse/vX.Y.Z-<commit hash>` by default
- Allow tweaking user agent via `--builder-user-agent "agent"`

lighthouse/tests/beacon_node.rs

@@ -716,6 +716,40 @@ fn builder_fallback_flags() {
     );
 }
 
+#[test]
+fn builder_user_agent() {
+    run_payload_builder_flag_test_with_config(
+        "builder",
+        "http://meow.cats",
+        None,
+        None,
+        |config| {
+            assert_eq!(
+                config.execution_layer.as_ref().unwrap().builder_user_agent,
+                None
+            );
+        },
+    );
+    run_payload_builder_flag_test_with_config(
+        "builder",
+        "http://meow.cats",
+        Some("builder-user-agent"),
+        Some("anon"),
+        |config| {
+            assert_eq!(
+                config
+                    .execution_layer
+                    .as_ref()
+                    .unwrap()
+                    .builder_user_agent
+                    .as_ref()
+                    .unwrap(),
+                "anon"
+            );
+        },
+    );
+}
+
 fn run_jwt_optional_flags_test(jwt_flag: &str, jwt_id_flag: &str, jwt_version_flag: &str) {
     use sensitive_url::SensitiveUrl;