Support for single component pixel format (gray) for alpha layer in avif

AVIF supports transparency via second stream in container that needs to be a single component (gray). AOM AV1 provides this support however it seems that SVT-AV1 does not.

To produce transparent AVIF, following FFmpeg command can be used:

ffmpeg -i source.webp -filter_complex "[0]split[v][v1];[v1]alphaextract[v1]" -map "[v]" -map "[v1]" -f avif -vcodec libaom-av1 -y out.avif

When the same command is used with SVT-AV1 the following error is printed:

[avif @ 0x15af35010] Second stream for AVIF (alpha) output must have exactly one plane
[out#0/avif @ 0x15af34f50] Could not write header (incorrect codec parameters ?): Invalid argument
Error while filtering: Invalid argument

Cross linking the FFmpeg ticket for the same issue https://trac.ffmpeg.org/ticket/10962