ktx file format does not show up in QtDS

.ktx assets do not show up in QtDS asset library, and cannot be imported to QtDS.

We have a project that uses .ktx files instead of .hdr files. The project runs fine, but the .ktx assets are not visible in QtDS asset library at all.
https://git.qt.io/tetammin/wow/-/tree/master/Wow

We should support .ktx files in QtDS as they are supported by Qt already, and projects targeting embedded HW need to be able to use compressed formats.