RIO-31 feat(really-core): Implement Reference Field Type 

- refactor CollectionActor to be FSM
- handle reference field on create request on CollectionActor
- fix test cases for CollectionActor
- add reference field
- implement CollectionActorTest to extend CollectionActor and handle test messages like `GetState`
- fix test cases for CollectionViewMaterializer, ModelLoader and GorillaEventCenter
Can't we use DataType[R] in the loader too?