switch settings to use pydantic-settings

[d-v-b]

This replaces the settings management with a pydantic-settings-based approach. For backwards compatibility, old APIs are added on top of the new pydantic-based foundation. Opening as a draft while I iterate on the implementation.

[d-v-b]

this is ready for review. I did not change how the config is used in the codebase. I recommend deprecating the mutable-mapping approach (config["database.password"] = new_password) because this is harder to make type-safe and also doesn't support autocomplete. A simpler API is probably something like config.database.password = new_password or config.database.set_password(new_password).

[dimitri-yatsenko]

I am okay with the settings not being backward compatible. This is a major release where we can break settings compatibility. The data will be backward compatible.