Allow to specify av1 software encoder

[Mr-Pi]

ffmpeg supports multiple av1 encoders(like libaom-av1, libsvtav1 or librav1e), but currently there is now way to specify which encoder should be used.

[bileslav]

Any updates or plans on this? The default encoder libaom-av1 is incredibly slow for me (same as libvpx[-vp9] btw); libsvtav1 works just fine.