Traits for Qt JSON

[dantti]

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Help with adding traits for Qt JSON types

Additional Context

I've been using JWT-CPP for a long time, prior traits, and using picojson isn't much of a pain, still since I'm alwayd on Qt related code being able to use Qt types simplifies the code, I feel like I managed to get a bit far, tho code does not compile.
It seems my main issues are:

• QString support, which the current traits always have a comment saying that there is not support for anything different than std::string
• object_type [] operator, this seems tricky as QJsonValueRef is returned to be able to have a value assigned for QJsonObject and QJsonArray, still the latter doesn't seem to require such operator

My current attempt is at cutelyst/FirebaseAdminQt#2 hopefully you can also see the build issues, would be great if this was possible, and upstreamed here.

Can I get some help?