add support for multi-audio-track YT-videos

[a-tze]

After years of waiting, YT finally has support for multiple audio tracks as it seems.

I have no idea if this feature is limited to some "high roller" accounts or if anyone can use it, and just stumbled upon it recently:

https://youtu.be/84uelyKOMmI?t=10

Supporting this could make the whole YT publishing a lot easier - but probably needs ways to advertise the feature inside the video, e.g. with a different intro or YT title something like that.

[titaniumtraveler]

I also just recently learned about this feature.
Initially at the time where youtube started adding AI generated localization everywhere (including title and decription!), which did go as well as one would expect ...

Today's video upload on the media.ccc youtube channel I was reminded of that and decided to look if they do allow for custom audio (which they do as @a-tze mentioned).
The official documentation for the feature is here: https://support.google.com/youtube/answer/13338784

It is a bit weird and the alternative audio tracks have to be uploaded as "Subtitles".

Based on that I found out that this applies to the youtube API as well, which means that the alternative audio tracks should be possible to upload via the Captions API.

See https://developers.google.com/youtube/v3/docs/captions
And https://developers.google.com/youtube/v3/docs/captions/insert