Support entities without class constructors

[mpareja]

Gearshaft's EntityStore and PostgresDocumentStore currently require entities to be implemented using classes. Class constructors are the expected value of the entity parameter when creating such stores.

Existing users would be familiar with the following entity store declaration:

createEntityStore({
  category: 'registration',
  entity: Registration,
  projection: RegistrationProjection,
  messageStore
})

The proposed change would instead require users to provide a function to instantiate entities. For class-based entities, the syntax would look as follows:

createEntityStore({
  category: 'registration',
  entity: () => new Registration(),
  projection: RegistrationProjection,
  messageStore
})

Note, the entity parameter now encapsulates how the entities are instantiated.

Reasoning

The proposed change enables more flexible construction of entities and lifts the requirement for all entities to be implemented as classes.

Plan

• [ ] PostgresDocumentStore: update entity parameter usage
• [ ] EntityStoreSubstitute: receives entity parameter usage
• [ ] EntityStore: receives entity parameter usage