Support entity types

Base worker entity mixin should list once the entities available on the corpus, and use their ID when an entity is referred by its name.

Depends https://gitlab.com/teklia/arkindex/backend/-/issues/1410

Refs https://redmine.teklia.com/issues/3186

A new method check_required_entity_types will also be available: it will check that all required entity types are available on a corpus, or create them (using their names).