QtProtobuf: add QML support

The protobuf patches originally had QML support, but it was taken out for time concerns when trying for the 6.4 feature freeze.
The support should be restored before release, this means restoring the QProtobufLazyMessagePointer's functionality to not deallocate objects owned by a qmlengine. And finalizing + merging the ProtobufQuick patch
https://codereview.qt-project.org/c/qt/qtdeclarative/+/412818

UPDATE:

The task is not about porting anymore due to big difference between curretnt GRPC version based on Qt6 and old one based  on Qt5.

I need to re-estimate it because code from existing review is not relevant anymore.