Support for annotation based entity callbacks

Entity callbacks (e.g. in MongoDB) are a powerful way of reacting to the persistence layer interacting with the entity. It would be cool if the entities could be annotated directly and get those methods called in the process of the callback execution in general.

The annotated model has the benefit of no needing an extra component being registered with the infrastructure while of course being limited to not being subject for dependency injection, access to the raw entity state source abstraction etc. but might be just enough for some use cases.

The issue names no files, tests, or entry points, so begin by locating the entity-callback infrastructure and its annotation processing conventions. Define what annotated callback methods should support and how they are discovered during callback execution, while preserving the stated limits around dependency injection and raw entity state access. Done means the feature works generally without registering an extra component.