Refactor conf system and cli

[adriendelsalle]

I would like to refactor both the configuration system and the cli to propose something more modular and leverage existing libs.

Motivations:

• replace the implementation of type checking, default value, required or not, section vs entry, by an existing lib doing that better
• have a modular and extensible config system that paves the way for having pluggable authenticators, storage, etc.
• benefit from a cli allowing by design args parsing and not only a config file

I think about traitlets that provides all of those features. It could be also very convenient to reuse/share some JupyterHub implementations.

There is also pydantic and the promising pydantic-cli. That way we reuse the already installed pydantic dep required by FastAPI. Looks like some jupyter projects are also assessing pydantic. See this issue.

I'm still surprised that traitlets never gained so popular that pydantic is. Is there some reasons explaining this difference of popularity that could help us to make a choice?

Do you have some thoughts about that @wolfv @SylvainCorlay ? @btel ? Any others?

[btel]

I agree that configuration system could be better. I would not like to introduce another type checking library, mainly because of cognitive load for developers to learn two different libraries. I don't know either traitlets or pydantic to compare their relative benefits.