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

Fixes #1422.

src/etl/impl/SourceImpl.hpp

@@ -34,6 +34,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 namespace etl::impl {
@@ -202,6 +203,7 @@ public:
      *
      * @param request The request to forward
      * @param forwardToRippledClientIp IP of the client forwarding this request if known
+     * @param xUserValue Optional value of the X-User header
      * @param yield The coroutine context
      * @return Response wrapped in an optional on success; nullopt otherwise
      */
@@ -209,10 +211,11 @@ public:
     forwardToRippled(
         boost::json::object const& request,
         std::optional<std::string> const& forwardToRippledClientIp,
+        std::string_view xUserValue,
         boost::asio::yield_context yield
     ) const final
     {
-        return forwardingSource_.forwardToRippled(request, forwardToRippledClientIp, yield);
+        return forwardingSource_.forwardToRippled(request, forwardToRippledClientIp, xUserValue, yield);
     }
 };