Trac ticket number

ticket-28586

Branch description

To support field fetching modes for GenericForeignKey, it needs to conform to a new "fetcher protocol", which includes having a field attribute that points from the "fetcher" to the actual field.

The core related fields, like ForeignKey, all use descriptors with a field attribute. These descriptors then act as their "fetcher" class.

GenericForeignKey has always been a bit odd because it does not have a separate descriptor class. This PR removes that oddity by adding such a class and moving the appropriate methods there.

This change follows ticket-35224, which made GenericForeignKey an actual field class.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.