[Youtube] Provide the mpd manifest for youtube livestreams

[FDMX2]

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Youtube serves livestreams as hls and mpd (MPEG-Dash). Thus far youtube-dl only supports hls livestreams.
I suggest youtube-dl should also provide the url for the mpd manifest since the youtube hls livestreams comes with less quality features (for example hls is capped to 30 fps).
In order to implement a the necessary mpd live downloader (like it exists for hls) first the mpd manifest url should be available to provide a way to get test streams.

A way to retrieve the mpd live manifest is shown here:
#15202

The live dash manifest URL is in the player config JSON.

youtube-dl --print-traffic https://youtu.be/<id> \
| grep -oP "/api/manifest/dash/.*? " \
| echo "https://manifest.googlevideo.com/$(cat -)"

https://manifest.googlevideo.com//api/manifest/dash/etc....

Or extract it yourself:

curl -sL "https://youtu.be/<id>" \
| grep -oP "ytplayer.config =\K.*?}(?=;)" \
| jq -r .args.dashmpd

https://manifest.googlevideo.com/api/manifest/dash/etc....

by @hmedia1