nix: add flake

[andrevmatos]

• add a flake.nix file to the repo
• builds rustica package with rust tools from nix
• will allow running rustica with nix run github:obelisk/rustica
• provides a devShell with nix with build tools and rustica itself
• provides an overlay to add rustica to the system packages
• provides a home-manager module, to enable rustica-agent user service in linux/systemd systems

home-manager module can be used in a flake like this:

{
  inputs.rustica.url = "github:obelisk/rustica";
  outputs = {self, nixpkgs, rustica, ...}: {
    home-manager.users.user = {pkgs, ...}: {
      imports = [rustica.hmModules.rustica-agent];
      services.rustica-agent = {
        enable = true;
        environment = "prod";
      };
    };
  };
}

[andrevmatos]

Needed due to #55

[obelisk]

Will try to address this in the next version