The configuration of an AiiDA instance is written in JSON format to the
`config.json` file. The schema is defined using `jsonschema` to take
care of validation, however, some validation, for example of the config
options was still happening manually.

Other parts of the code want to start using `pydantic` for model
definition and configuration purposes, which has become a de-factor
standard for these use-cases in the Python ecosystem. Before introducing
another dependency, the existing `jsonschema` approach is replaced by
`pydantic` in current code base first.