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

Fixes #1422.

src/etl/impl/ForwardingSource.hpp

@@ -28,6 +28,7 @@
 #include <chrono>
 #include <optional>
 #include <string>
+#include <string_view>
 
 namespace etl::impl {
 
@@ -49,6 +50,7 @@ public:
      *
      * @param request The request to forward
      * @param forwardToRippledClientIp IP of the client forwarding this request if known
+     * @param xUserValue Optional value for X-User header
      * @param yield The coroutine context
      * @return Response wrapped in an optional on success; nullopt otherwise
      */
@@ -56,6 +58,7 @@ public:
     forwardToRippled(
         boost::json::object const& request,
         std::optional<std::string> const& forwardToRippledClientIp,
+        std::string_view xUserValue,
         boost::asio::yield_context yield
     ) const;
 };