Problem/Motivation
This is a sister issue of #2517030: Add a URL formatter for the image field and #2825487: File entity should expose the file URL as a computed base field. The former issue solved it for image entities exposed via Views REST export displays. This issue must solve it at the serialization/normalization level, so that all REST resources (as well as JSON API) benefit.
The latter issue already is doing it for FileItem (and hence ImageItem because it extends FileItem), but only for the "canonical" file URL. In the case of ImageItem, we also want image style URLs.
Proposed resolution
Add ImageItemNormalizer.
Remaining tasks
TBD
User interface changes
None.
API changes
TBD
Data model changes
TBD