refactor: improve type extensibility and its safety (#348)

tests/stresstests/sdbus-c++-stress-tests.cpp

@@ -427,8 +427,8 @@ int main(int argc, char *argv[])
                 while (!stopClients)
                 {
                     std::map<std::string, sdbus::Variant> param;
-                    param["key1"] = "sdbus-c++-stress-tests";
-                    param["key2"] = ++localCounter;
+                    param["key1"] = sdbus::Variant{"sdbus-c++-stress-tests"};
+                    param["key2"] = sdbus::Variant{++localCounter};
 
                     concatenator.concatenate(param);