Passed ResultT down to engine plugins, so now they can form replies in different formats.

include/engine/api/base_result.hpp

@@ -0,0 +1,21 @@
+#ifndef ENGINE_API_BASE_RESULT_HPP
+#define ENGINE_API_BASE_RESULT_HPP
+
+#include <mapbox/variant.hpp>
+
+#include <string>
+
+#include "util/json_container.hpp"
+
+namespace osrm
+{
+namespace engine
+{
+namespace api
+{
+    using ResultT = mapbox::util::variant<util::json::Object, std::string>;
+} // ns api
+} // ns engine
+} // ns osrm
+
+#endif