[Feature Request] Add ffv1 as encoder

[EdgeLord69-dev]

as the title states, i hope that ffv1 could be added to av1an as an available encoder. Since its a lossless codec, it wouldnt require any target quality probing while speeding up encoding to a lossless ffv1 output

[shssoichiro]

I wonder how much gain this would provide. It wouldn't need SCD and could just use fixed chunk sizes, my biggest thought is that if disk IO would become the bottleneck, then chunking wouldn't provide much benefit.

It could be worth evaluating, at least.