Add an AVIF muxer by re-using the existing the mov/mp4 muxer.

AVIF Specification: https://aomediacodec.github.io/av1-avif

Sample usage for still image:
ffmpeg -i image.png -c:v libaom-av1 -still-picture 1 image.avif

Sample usage for animated AVIF image:
ffmpeg -i video.mp4 animated.avif

We can re-use any of the AV1 encoding options that will make
sense for image encoding (like bitrate, tiles, encoding speed,
etc).

The files generated by this muxer has been verified to be valid
AVIF files by the following:
1) Displays on Chrome (both still and animated images).
2) Displays on Firefox (only still images, firefox does not support
   animated AVIF yet).
3) Verified to be valid by Compliance Warden:
   https://github.com/gpac/ComplianceWarden

Fixes the encoder/muxer part of Trac Ticket #7621
Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>