Problem/Motivation
When reusing the same image in different places, the alt text of the image might be different for each use. Since images could contain a lot of elements, you might want to describe different parts of the image depending on the context of how the image is used. It is currently not possible to override the canonical alt text of a media image on a per use basis. (Similarly, different tags might be needed depending on where it is used.)
Proposed resolution
Add an map field to the media entity reference to store the field overrides. Note the override is per fieldand not per field_property.
Remaining tasks
Add more doxygen to the media item class, especially around the per field vs per property override.