Add support for AVIF to coverstore

These next-gen image formats offer great compression ratios over traditional jpg. It would not only be smaller on disk, but would improve page load times of open library.

### Describe the problem that you'd like solved

### Proposal & Constraints

### Additional context
Quote @onnotasler :
> As far as I see, we are using JPEG as image file format at the moment. Would it be worthwhile to consider a switch to AVIF? As far as I am aware, most modern browsers support AVIF graphic file format nowadays and it offers similar image quality at lower image size.
>
> Here is an example of a workflow to improve size to quality even more:
https://www.tempertemper.net/blog/avif-and-webp-are-not-always-better-than-png-and-jpg

AVIF Support is ready for Pillow as of 2025! https://github.com/python-pillow/Pillow/issues/7983

### Stakeholders
@onnotasler