youtube.lua: &fmt={audio,video} URL parameters for "adaptive" formats

This extends the old &fmt=[itag] URL syntax that we still supported all
along to force format selection, and allows choosing from "adaptive"
elementary streams without knowledge of itag specifics and in accordance
with normal resolution preferences.

This also allows playing only the audio part of music videos and
skipping the download of the video part entirely, greatly reducing
bitrate and providing mitigation against the throttling issue.

Ref #10237, #27227