Add Media Field for Album

[gcoppola19]

Fixes 6092

Add a media field for album, when a user runs beets fields, media will be listed for album. This way, media of album can be kept track of.

Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
[x ] Tests.
test_media_field.py

[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[henry-oberholtzer]

Hi Grace! Thanks for the PR and your interest in contributing to Beets!

We currently have the media field provided by the Item class, which represents an individual song or track, which currently can be queried on an album level, and allows support for albums to have tracks from different media types (e.g. tracks from both CD and DVD ripped to digital file).

If there's a use case that isn't addressed by the current implementation, more context & background for the issues this addresses would be appreciated!

Thanks!