ImageItem should have an "derivatives" computed property, to expose all image style URLs

Problem/Motivation

This is a sister issue of #2517030: Add a URL formatter for the image field → and #2825487: Fix normalization of File entities: file entities should expose the file URL as a computed property on the 'uri' 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

None.

User interface changes

None.

API changes

Adds an API to allow explicit bubbling of cacheability metadata: pass in a 'cacheability' => instanceof CacheableMetadata key-value pair in the serialization context.

(The ImageItem normalizer that we add here uses this to its advantage: this new capability allows that normalizer to be properly unit-testable. No more global state manipulation! See #67's interdiff.)

Data model changes

None.