Added support for AVIF image uploads

We've supported AVIF output through image transformations, but now you can also upload AVIF images directly to Sanity. No more converting modern image formats before upload—our image pipeline now accepts AVIF files and automatically handles transformation to other format you need, just like other image types.

Note: This support is for 8bit AVIF only. We do not currently support 10bit AVIF uploads.