Make it easier to serve provider fields with validation

[ml-evs]

Following the discussion from #925, we should make it easier to extend our models and make sure they get used for validation in the API.

Currently, this requires changing the associated endpoint, entry collection and mapper code to get working (as many have done in our forked implementations). This should be something that is configurable, presumably via something like: "data_models": {"structures": MyStructureModel} in the config (which can then set the appropriate entry info schemas, mapper resource class, collection response class etc.).