[C++][Qt]Replace usage of QVariant with a more intuitive optional wrapper (#8960)

* Replace usage of QVariant with a more intuitive wrapper which also supports non standard types/classes

* Fix crash due to usage of object instead of reference

samples/client/petstore/cpp-qt5/PetStore/PetApiTests.cpp

@@ -217,6 +217,7 @@ void PetApiTests::updatePetWithFormTest() {
     // fetch it
     bool petUpdated2 = false;
     connect(&api, &PFXPetApi::getPetByIdSignal, [&](PFXPet pet) {
+        Q_UNUSED(pet);
         petUpdated2 = true;
 //      QVERIFY(pet.getName().compare(QString("gorilla")) == 0);
         QTimer::singleShot(0, &loop, &QEventLoop::quit);