Discogs: artist name variation support + artist credit support. (#6050)

Implements #3354, updates #6040. This PR also improves typehints for the discogs plugin.

The name variations provided by Discogs can now be used by the
auto-tagger through 3 config options.
By default, the plugin will write the variation to the tag
`artist_credit`, but through three config options can also write it
to the album artist tag, the track's artist tag, or any combination of
the three.

This PR contains a small addition on #6040, letting the string used to
join featured artists be customized.

The new configuration options available, and their defaults, are as
follows:

```yaml
discogs:
  featured_string: "Feat."
  anv:
    artist_credit: True
    album_artist: False
    artist: False
```

Author: semohr