[FEATURE] Add support for JPEG-XL

This commit adds support for the following features:

- RGB, Gray and CMYK U8/U16/F16/F32
- Animated JXLs r/w
- Exif, XMP, and IPTC metadata r/w
- Parallel encoding & decoding (there's a step that isn't parallelized
  on libjxl yet, though)

Note that:
- for F16 images, the import plugin will ask libjxl to return
a F32 image in order not to depend on OpenEXR.
- JXL is a single-layer format, the export will
use the document projection-- individual layers will be lost.

This is the first plugin to introduce full native animation import &
export in Krita.

CCMAIL: kimageshop@kde.org