Add support for use with JSON:API

Support for using this module with JSON:API is in progress but not yet in a working state.

To make it easier to track what remains to be done, I'm opening this meta issue.

At present, the fundamental issue is that the core JSON:API module's handling of entity references is tightly coupled to Core entity references, and it must be refactored to provide a new API to allow modules such as this one to provide serializers for use with JSON:API.

Issue currently blocking all the others: #3057545: ResourceTypeRepository wrongly assumes that all entity reference fields have the setting "target_type"

Issues: This module

#3057556: Implement DERItem::getReferenceableBundles
#2844946: Add DynamicEntityReferenceItemNormalizer for serialization module

Issues: Core

#3057545: ResourceTypeRepository wrongly assumes that all entity reference fields have the setting "target_type"
#3027430: Dynamic Entity Reference normalization support
#3277553: JSON:API module should support POSTing dynamic entity references with only `type`, `id`/entity reference field should validate type when setting with entity property
#3277766: Throw more helpful exception when denormalizing unrecognized field properties
#3029090: Dynamic Entity Reference: support discovering relatable resource types