[Feature request] Build audio/webm and video/webm streams in DASH manifest

[ndygo]

Is your feature request related to a problem? Please describe.
As part of #3011, the audio/webm and video/webm streams have been removed from the manifest build because of a videojs issue. This means that the VP9 and Opus streams are not used by clients.

I can personally tell whether I am listening to a YouTube AAC stream because of the audio quality in comparison with Opus. In addition, the VP9 streams on YouTube are higher quality than the H.264 streams, and there isn't always AV1 streams (which can be contained in mp4).

Describe the solution you'd like
I'd like to see a solution like unixfox originally suggested here: #3011 (comment)

Where if the client requesting the manifest is not videojs, the audio/webm and video/webm streams are built. Perhaps this could be done by appending a parameter to the URL given to videojs.

Describe alternatives you've considered
None.