ContentModerationStateInterface additions

Problem/Motivation

The ContentModerationState implements the ContentModerationStateInterface, but that interface is empty.

The public custom methods should be part of that interface. It would also be nice to have a method to retrieve the moderated entity, since that must be done now by directly accessing the content_entity_id value.

Proposed resolution

Add a new method getModeratedEntity(), and add the public methods to the interface.

Remaining tasks

User interface changes

API changes

Data model changes