prepro: refactor config to improve typing #5770
Description
As discussed with @corneliusroemer during the multi-path changes we should improve the prepro config.
Main issues:
- Complicated parsing and validation of types, this can be done by pydantic without custom rolling any code
- Complicated creation and passing or args and env variables: this can be done with pydantic-settings: https://docs.pydantic.dev/latest/concepts/pydantic_settings/#parsing-environment-variable-values
- Mutability of config, pydantic's
@model_validator(mode="after")can be run after creating a config to modify the config in one specific place with validation.