Add option to separate the app's volume control from Android main volume

Checklist

[X] I made sure that there are no existing issues - open or closed - which I could contribute my information to.
[X] I have read the FAQ and my problem isn't listed.
[X] I'm aware that this is a request for NewPipe itself and that requests for adding a new service need to be made at NewPipeExtractor.
[X] I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
[X] This issue contains only one feature request.
[X] I have read and understood the contribution guidelines.

Feature description

Also described in this Stack Exchange post.

An option in the Settings so that the volume of Newpipe's output is decoupled from the Android main volume.

MXPlayer has something very similar in Settings > Audio > Disable the system volume. But this needs the file to be locally downloaded, seems to work with video files only,

Why do you want this feature?

My usecase is listening to some beats from YT, while watching lecture videos on the Youtube app or cloned NewPipe app.

I'm using an older Redmi device, which is running MIUI 12.5 but does not come with Sound Assistant, and is not rooted (rooting is not an option). I've used Shizuku + App Ops to turn off Audio Focus for the needed apps, so now I can play a mixtape on NewPipe and the lecture video on the Youtube app. But I cannot adjust the volumes for the two separately, so the music drowns out the instructor's voice in the lecture.

The only solution it seems is a YT frontend which has its own volume control i.e. the volume control in the app controls the volume of the output it sends to the main audio channel, rather than controlling the device volume.

Additional information

My current solution is using SoundAura from F-droid for the background music — it has its own internal volume control as needed; but it requires me to download the mixtape from YT into local as an mp3 (which I do with NewPipe), and I can't skip forward or backward on playback. SoundAura is open source on Github and written entirely in Kotlin, so I'm hoping importing this feature into NewPipe from their codebase wont be too much of hassle.

TeamNewPipe / NewPipe