API for JSON:API specific "extra" fields, e.g. for entity labels

Problem/Motivation

The concept of a display_name field was added to all user resources in JSON:API responses in #3057175: Implementation of user name in JSON:API can result in overwriting data → . It contains the result of UserInterface::getDisplayName(). Rather than having a special carve-out for this field, it would be interesting to consider a generalized feature to support "extra fields" on JSON:API results.

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

None.

API changes

TBD

Data model changes

None.

Release notes snippet

TBD