Extend the Property metatype to allow a type that is an EntityType.

This proposal extends Property to allow a type declared as an EntityType or Singleton.
This is different from a NavigationProperty with ContainsTarget=True, in that the contained entities are not separately addressable; (either as a property of the EntityContainer or via its key property values and its EntitySet.)

Using an EntityType in a property declaration is useful when storing the value of an Entity or collection of Entities, such as in a log or event record.