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.

[pyansys-automation]

Hello @FedericoNegri, @ojkoenig!
This issue has been open for over a year and has had no updates in the past 8 months. If it is still relevant, please provide an update. Otherwise, consider closing it to help keep the issue tracker clean.

If this issue needs to remain open, please comment below with @pyansys-automation never close issue to prevent it from being automatically closed.

If you want this repository to be excluded from this automated maintenance process, please let us know by filling in the opt-out request form.