Add nix service definition for codex

[markoburcul]

Added service definition for Nim Codex and tests.
The service can be instantiated by including it in flake inputs:

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
    codex-flake.url = "git+https://github.com/codex-storage/nim-codex?submodules=1#";
  };

and configuring the service:

services.nim-codex = {
   enable = true;
   settings = {
       data-dir = "/var/lib/codex-test";
   };
};

[jakubgs]

I think you've over-done it with defining every single setting, but it's not a bad thing :D.

Also, a README might benefit from an example of how to use the service.

[jakubgs]

Readme doesn't have an example of how to load the service module.