QtRemoteObjects: Investigate possibility of supporting different serialization backends

Investigate what changes need to be done to support other serialization backend(s) apart from QDataStream, to be able to support also non-Qt clients. Identify:

• what kind of API changes are required
• would there be any binary compatibility breaks
• do the changes need to be done with Qt 6.2 release, or they can be done also later?

From conversation with Brett: 

The current codebase uses QDataStream to serialize types for over-the-wire transfer.  This would at least need to be updated for changes from Qt5 to Qt6 (if any).  However, this ties in to registration of types, and can be a pain point.  I'm looking at what can be done with COBS CBOR as the underlying protocol.  This would have a significant benefit of making not-Qt clients possible, at least for a subset of types.

Potentially relevant patches might be: https://codereview.qt-project.org/q/topic:%2522serialization%2522+project:qt/qtremoteobjects