youtube.lua: generic support for YouTube "adaptive" formats

Formats listed under that label are audio-only or video-only elementary
streams, but offer choice encompassing the full array of supported
resolutions, qualities and codecs; whereas classic multiplexed formats
have long been dwindling down to only two formats now, 720p and 360p, or
even 360p only for some content.

For now, these "adaptive" formats are only used if explicitly requested
by itag number.

Ref #10237