youtube: include whether the video contains paid promotion in the info json

[CounterPillow]

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• Single video: https://youtu.be/9Bkcd05IQpE

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Description of your issue, suggested solution and other information

YouTube apparently has a flag for uploaders to specify whether a video includes paid promotion. If it does, the YouTube web player will show a small banner in the bottom left saying "Includes paid promotion" when the video is first loaded. youtube-dl --write-info-json and youtube-dl -j does not contain this parameter, but it would be nice if it did.

The purpose of this flag is to be in compliance with various advertising laws around the globe; for e.g. video players which use youtube-dl to play back YouTube content, being able to check whether a video contains paid promotion would allow them to show viewers a similar notice as the YouTube web player does.