Implement observable generic container

[GerkinDev]

In order to implement relations, entity relation fields should be an observable field, that runs the correct query if required when trying to access a property.

Example use case:

const entity: Entity = await myModel.find({id: 1});
const subEntity = await entity.attributes.relationalT1Field;// subEntity is an Entity
const subSet = await entity.attributes.relationalTxField;// subSet is a Set