Changed requirements for images in OPDS 2.0

[HadrienGardeur]

Based on @llemeurfr initial comment on #52 :

• Lowered the requirement for images in publication to a SHOULD
• Dropped requirement from the JSON Schema
• Added AVIF support in spec and JSON Schema

[HadrienGardeur]

While the initial issue opened by @llemeurfr didn't mention AVIF, I used this opportunity to add it to the list of supported formats.

AVIF is now widely supported across browsers and it's a strong contender for replacing JPEG as the default format for covers in OPDS now.
WebP is another potential addition to this list, but I'm not sure that it makes a whole lot of sense now that AVIF is well supported.

[barmintor]

For what it is worth: I think the spec would benefit from being silent about required image formats for thumbnails:

• The ability to display an image of a given format is an out-of-band client concern
• The images property/collection is not required for correctness
• The spec includes MIME information for the links for a client to decide whether it can make use of the link
• There's no difference for the reader between there not being a supported image and there not being images at all