Add "music_in_this_video" metadata if it's present, similar to "chapters"

[kylebebak]

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Description of your issue, suggested solution and other information

YouTube now has a "Music in this video" section in the video info, for example in this video.

It seems like youtube-dl currently has no way the data in this section.

If it's possible, I think it would be excellent if youtube-dl could include this info under a "music_in_this_video" key, for example when running youtube-dl --skip-download --write-info-json yE5xi2uLIHs.

"music_in_this_video" could contain an array of objects with info about each track, like "chapters".