C API: Validate with nixos-option package

[roberth]

Is your feature request related to a problem? Please describe.

The C API should be/become good enough for use by the nixos-option package.
By using it in nixos-option, we validate the C API.

Additional context

This issue is mostly a reminder for in the C API milestone. Nixpkgs issue:

• nixos-option: use the Nix C API, reduce maintenance burden nixpkgs#355914

Priorities

Add 👍 to issues you find important.

[edolstra]

I'm thinking the other way: nixos-option (or the core of its functionality) should become part of Nix, ideally using flake schemas. We should have something like a nix show command that displays all the configuration options in a flake output, such as a NixOS configuration.

[roberth]

That's an interesting idea for an alternative, but not something to block this (mostly maintenance) issue on, and technically that's a decision that's up to NixOS maintainers.