Add av1_nvenc as selectable codec

Merged to ffmpeg a long time ago and to Discord a moment ago, users of RTX 4000 series can enjoy blazing fast AV1 encoding. Tested it out on plain ffmpeg, 300ish FPS on 1440p60 footage using -c:v av1_nvenc.

Inserting such codec (with manually updated ffmpeg bin) to a profile yields error about unsupported codec: