Add image attachments support using Qt

[grynspan]

This issue tracks adding the equivalent of AttachableAsCGImage when using Qt. We presumably would want to add support for QImage using QImage::save().

Problems to solve:

• Qt is not guaranteed to be available, so needs to be weak-linked (?)
• Qt is not, AFAIK, Clang-modularized
• Qt is C++ which makes interoperating with it more difficult

Relevant to our interests: https://www.qt.io/blog/about-the-new-qt-bridging-technology

[grynspan]

Qt PR to add module maps: https://codereview.qt-project.org/c/qt/qtbase/+/674683