Add option to set X-User header value for forwarded requests (#1425)

Fixes #1422.

tests/common/util/MockLoadBalancer.hpp

@@ -40,7 +40,7 @@ struct MockLoadBalancer {
     MOCK_METHOD(
         std::optional<boost::json::object>,
         forwardToRippled,
-        (boost::json::object const&, std::optional<std::string> const&, boost::asio::yield_context),
+        (boost::json::object const&, std::optional<std::string> const&, bool, boost::asio::yield_context),
         (const)
     );
 };