Add image-rs integration

[RReverser]

Often enough, one just wants to read/write FITS as an image. For that, it would be nice to provide integration with the image crate (most popular library for images in the ecosystem).

They've recently added a plugin API that should make this pretty straightforward. Some real-world examples of integration PRs for other formats:

• Add image crate decoding hook tirr-c/jxl-oxide#480
• Cykooz/libheif-rs@9f366f6