Add integration with nixpkgs-terraform

[oscar-izval]

This PR adds integration with nixpkgs-terraform. To test this:

1. Copy this demo config for devenv and change <abs-path-to-devenv-clone> to the absolute path of your local checkout of this branch (with three /):

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
    systems.url = "github:nix-systems/default";
    devenv.url = "git+file://<abs-path-to-devenv-clone>?ref=add-nixpkgs-terraform";
    nixpkgs-terraform.url = "github:stackbuilders/nixpkgs-terraform";
  };

  nixConfig = {
    extra-trusted-public-keys = "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw= nixpkgs-terraform.cachix.org-1:8Sit092rIdAVENA3ZVeH9hzSiqI/jng6JiCrQ1Dmusw=";
    extra-substituters = "https://devenv.cachix.org https://nixpkgs-terraform.cachix.org";
  };

  outputs = { self, nixpkgs, devenv, systems, ... } @ inputs:
    let
      forEachSystem = nixpkgs.lib.genAttrs (import systems);
    in
    {
      devShells = forEachSystem
        (system:
          let
            pkgs = nixpkgs.legacyPackages.${system};
          in
          {
            default = devenv.lib.mkShell {
              inherit inputs pkgs;
              modules = [
                {
                  languages.terraform.enable = true;
                  languages.terraform.version = "1.7.0";
                }
              ];
            };
          });
    };
}

2. Run NIXPKGS_ALLOW_UNFREE=1 nix develop --impure and play around setting different versions