Consider replacing Marshmallow with Pydantic for REST models

Reasons:
* `marshmallow` requires splitting models into schemas and actual resources
* `pydantic` has an interesting [code generator](https://pydantic-docs.helpmanual.io/datamodel_code_generator/) we could use to auto-generate (some of the) models from the openapi spec
* `pydantic` has native support for typehints

Possible critical points:
* We currently rely on marshmallow.missing to mark undefined fields (either not specified client-side or not returned by the server for a certain request). How to best implement this concept in pydantic? See e.g. https://github.com/pydantic/pydantic/issues/1223 and https://stackoverflow.com/questions/66229384/pydantic-detect-if-a-field-value-is-missing-or-given-as-null
* Auto-generated models probably wouldn't cover well some of the "polymorphic" resources we have, like parameter definitions. We might need to handle those manually.

Start by reviewing the current Marshmallow schemas and REST resources, then compare their missing-field behavior with the Pydantic references linked in the issue. Examine whether the OpenAPI code generator can cover the existing models, especially polymorphic resources such as parameter definitions. Done would require an agreed migration approach that addresses these gaps.

Written by the indexing model from the issue text.