support lazy loading references for convenient recursive relationship definitions

[uladkasach]

i.e., complete of branch https://github.com/uladkasach/schema-generator/tree/features/lazyRef

    it('should be possible to declare an entity with a recursive / hierarchial relationship', () => {
      const person: Entity = new Entity({
        name: 'person',
        properties: {
          birthday: prop.DATETIME(6),
          mother_id: prop.REFERENCES(() => person),
          father_id: prop.REFERENCES(() => person),
        },
        unique: ['birthday', 'mother_id', 'father_id'],
      });
    });

will need to defer evaluation of fn's until after entity is "initialized" - meaning we probably want an internal and external representation of "Entity":

• internal must have properties defined completely
• external expects properties to be defined w/ lazy load and completely

and then during the normalization step we simply cast each "external" entity into the internal entity, by "evaluating" all of its properties

[uladkasach]

note: this is not a feature blocking issue as recursion is still currently supported by simply "appending" the properties to the entity after definition:

const person = new Entity({
  name: 'person',
  properties: {
    birthday: prop.DATETIME(6),
  },
  unique: ['birthday', 'mother_id', 'father_id'],
});
person.properties.mother_id = prop.REFERENCES(person);
person.properties.father_id = prop.REFERENCES(person);

its just ugly