Obtain Spotify track (popularity and audio features) attributes

[arsaboo]

This PR introduces a new command beet spotifysync to obtain all the audio features available for a track. This includes the popularity information and audio_features. The following audio_features are included: acousticness, danceability, energy, instrumentalness, key, liveness, loudness, mode, speechiness, tempo, time_signature, valence. See details here.

This features only processes tracks that already have the spotify_track_id (which was introduced in #4348). So, the workflow will be to run beet import which will populate the spotify_track_id. This information can then be used to run beet spotifysync.

Fixes #4347 #3578 #4094

Config required:

spotifysync:
    client_id: SPOTIFY_CLIENT_ID
    client_secret: SPOTIFY_CLIENT_SECRET
    force: no

My hope is that we get the initial PR so that folks can start using it and improve upon it.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Nice; thanks for getting this started! What do you think about rolling this new command directly into the existing Spotify plugin? That could avoid a bunch of the duplicated code and allow users to share the token between the two bits of functionality, for example.

[arsaboo]

I am not 100% versed with the existing Spotify plugin. It may be possible to combine the features, but I am a little hesitant. The existing plugin is written for the auto-tagger and serves the basic purpose of getting the tag information from Spotify.

Also, not everyone may require all these features (although that can be addressed with a config option). Unless you are completely opposed to this idea, it may be a good idea to keep it separate so that additional features can be added here.

[arsaboo]

Also note that it will make the importer significantly slow due to the timeouts and the additional processing required.

[sampsyo]

Ah, to be clear, I just meant adding this command to the Spotify plugin. That would mean you'd still have to run the command explicitly to fetch the new data; it would not run as part of the import. In the code, this is just a matter of defining the commands method of the existing plugin appropriately.

I don't have a strong opinion about whether it's one or two plugins per se, but I do strongly prefer not to duplicate code... and keeping it in one plugin seems most likely to avoid that duplication.

[arsaboo]

Is there any plugin template that I can follow?

[arsaboo]

closing in favor of #4352