Add plugin for JPEG XL (JXL)

This plug-in provides support for another next-gen image format called JPEG XL

Plug-in uses libjxl library.

Additional test files: https://github.com/libjxl/conformance/tree/master/testcases

More info at community website: https://jpegxl.info/

The KIMAGEFORMATS_JXL option (default ON) is just for emergency case to disable the plug-in (in case there is some breaking change in future versions of libjxl)

Please review/test.

The code is based on my qt-jpegxl-image-plugin