Add AnnotationPage reference

[digitaldogsbody]

Per https://iiif.io/api/presentation/3.0/#53-canvas, an AnnotationPage inside the items or annotations property can be referenced/external, but this is currently not validated properly by the schema, as it would be possible to add an external AnnotationPage that contained items

I think that to fix this we need an AnnotationPageRef in line with the other new reference types.