Extra validators for Canvas/HWD properties

[digitaldogsbody]

The specification as written says

A Canvas MUST have a rectangular aspect ratio (described with the height and width properties) and/or a duration to provide an extent in time.

Although the JSON Schema validates this (https://github.com/IIIF/presentation-validator/blob/main/schema/iiif_3_0.json#L635-L643) with anyOf hwd and making h&w dependent on each other - this validation does not come through when the Pydantic model is generated by datamodel-code-generator and all three fields end up being wrapped in Optional.

We could build a custom validator for this (https://docs.pydantic.dev/usage/validators/) and add it to the class with the new skeleton modification script

[glenrobson]

Discussed this on the call 13th Jan we think this would be useful and could be injected with the skeleton agenda.