Consider replacing Marshmallow with Pydantic for REST models

[FedericoNegri]

Reasons:

• marshmallow requires splitting models into schemas and actual resources
• pydantic has an interesting 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. How to have an “optional” field but if present required to conform to non None value? pydantic/pydantic#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.