Feature request: Support AVIF format in PhotoCache

[Peque]

It seems like CardDAV's PhotoCache is currently supporting PNG, JPEG, GIF, ICO and (recently) WebP images.

AVIF is the only well-supported commonly used format for still images that is missing (excluding SVG for security concerns).

AVIF is supposed to supersede WebP with better compression, and already has wide adoption across major browsers.

Nextcloud intercepts GET requests on addressbook URLs ending with ?photo, and uses ALLOWED_CONTENT_TYPES to set the type name, returning 404 if the content type is not supported (not part of the allowed content types struct). So it would be great to add support for this format inside PhotoCache in order to be able to use AVIF in contact photos.