Support side-loading object model payload definitions

[leonhard-s]

As part of moving to generated Pydantic classes (#63), the object model loader should be extended to support loading custom data classes. This would allow client code to generate their own data classes based on the OpenAPI spec and load it at runtime, rather than coupling the API wrapper code to the object model.

Auraxium should still provide its own baseline object model at the time of release, but the OpenAPI spec will always be easier to update than Auraxium.

This would also allow using Auraxium with third-prty APIs such as Sanctuary.Census, which shares the query syntax but uses a different object model.