Add getMixUrl method to StreamExtractors

[Stypox]

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

I added the getMixUrl(MixType) method to every StreamExtractor, along with an enum for mix types (video, music, channel, album), following TeamNewPipe/NewPipe#3583 (comment). This PR is a draft and still needs work, also the frontend UI is still a TODO, but feel free to give feedback ;-).

• Youtube video with mix: https://www.youtube.com/watch?v=l-mPoJe8wZQ
• Youtube video mix: https://www.youtube.com/watch?v=l-mPoJe8wZQ&list=RDl-mPoJe8wZQ
• Youtube music mix: https://music.youtube.com/watch?v=l-mPoJe8wZQ&list=RDAMVMl-mPoJe8wZQ
• Youtube channel mix: https://www.youtube.com/watch?v=l-mPoJe8wZQ&list=RDCMUCf5q0cbFOLbphljteZ9d4Pw (I don't know why it doesn't work, this needs investigation) https://www.youtube.com/watch?v=9iHM6X6uUH8&list=RDCMUC_aEa8K-EOJ3D6gOs7HcyNg
• Soundcloud track with station (click the More button): https://soundcloud.com/jim-yosef/jim-yosef-firefly-ncs-release
• Soundcloud station (still unsupported by the extractor): https://soundcloud.com/stations/track/jim-yosef/jim-yosef-firefly-ncs-release
• Bandcamp track with related album (in NewPipe just considered as a playlist): https://jimyosef.bandcamp.com/track/dawn

Fixes TeamNewPipe/NewPipe#3583
Fixes TeamNewPipe/NewPipe#3250

[Stypox]

@XiangRongLin YouTube mixes seem to work randomly: for some videos they exist, for some others they do not. I'd switch to the other approach of returning a MultiInfoItemsCollector from streamInfo.getRelatedItems(), with available mixes and all other related videos. The only problem here is that strings cannot be translated, but I'd just use the mix name provided by youtube and call it a day (e.g. "Mix – NoCopyrightSounds"), since that's the name used in MixPlaylistExtractor too.

[opusforlife2]

Yay! Youtube Music mixes! 💃

[opusforlife2]

Youtube Music mixes...

[Stypox]

Closing in favor of #788