Images: Support AV1 Image File Format (AVIF)

[lastzero]

As a user with AVIF images, I want PhotoPrism to convert these files to JPEG and index them so I can browse them as part of my picture library.

Also adds a new configuration option for the Sips file format converter on macOS, since older versions do not support AVIF:

Environment	CLI Flag	Default	Description
PHOTOPRISM_SIPS_BLACKLIST	--sips-blacklist	avif,avifs	do not use Sips to convert files with these EXTENSIONS

[lastzero]

PhotoPrism now offers read-only support for AVIF images. Until we have native support, they will be converted via Darktable, so colors and brightness may not be ideal. RawTherapee does not seem to support AVIF yet.