Originally requested by okno at https://github.com/oknozor/musicbrainz_rs/pull/70#issuecomment-973844792.

Entity attributes are manually documented for humans only, usually on the entity documentation page, for example https://musicbrainz.org/doc/Series#Type.
Attributes for a given entity are already returned with name and MBID in API responses. But the API doesn’t provide neither more info about these attributes (description, hierarchy…) nor the full list of these attributes. So it seems useful to provide lookup endpoints (or what we did for genres) for these attributes.

Here is the full list of attributes currently editable by admins at least:

1. AreaType
2. ArtistType
3. CollectionType
4. CoverArtType
5. EventType
6. Gender
7. InstrumentType
8. LabelType
9. Language
10. MediumFormat
11. PlaceType
12. ReleaseGroupType
13. ReleaseGroupSecondaryType
14. ReleaseStatus
15. ReleasePackaging
16. Script
17. SeriesType
18. WorkType
19. WorkAttributeType