Retrieves the managed reference Id for an object that is referenced using SerializeReference.
Unity assigns an Id for an object when the Unity Object referencing the object has been serialized, or when SetManagedReferenceIdForObject is called.
If no Id has been assigned yet, this method returns SerializationUtility.RefIdUnknown.
See Also: SetManagedReferenceIdForObject, GetManagedReferenceIdForObject, SerializeReference.