Prevent discogs genre overwrite / Exclude fields on import

Discogs is a useful source for meta-data, however, the genre information in Discogs is pretty useless. Currently, when we use Discogs as a tag source, everything is overwritten by Discogs information. I posted it on [Discourse here](https://discourse.beets.io/t/prevent-discogs-plugin-from-overwriting-existing-genre-tags/1884) and Adrian suggested that currently, the Discogs plugin is not configurable.

In general, I think it would be a good idea to make plugins configurable so that they don't overwrite other details that the user may not want.

### Proposed solution

Make Discogs plugin configurable so that users can choose fields to be ignored. The current behavior (where it imports everything) can be the default so that this is not a breaking change.

### Objective

Avoid getting unwanted information from Discogs.

#### Goals

Provide more fine-grained user control.

Start with the Discogs plugin and its import field-handling and configuration behavior; the issue does not name specific files or tests. The work is done when users can configure fields to exclude while the default still imports everything, with coverage for the configured and default behaviors.

Written by the indexing model from the issue text.