nixos/knot: also allow config by YAML file

  in result;

  configFile = pkgs.writeTextFile {

  configFile = if cfg.settingsFile != null then

    assert cfg.settings == {} && cfg.keyFiles == [];

    cfg.settingsFile

  else pkgs.writeTextFile {

    name = "knot.conf";

    text = (concatMapStringsSep "\n" (file: "include: ${file}") cfg.keyFiles) + "\n" + yamlConfig;

    # TODO: maybe we could do some checks even when private keys complicate this?

        '';

      };

      settingsFile = mkOption {

        type = types.nullOr types.path;

        default = null;

        description = lib.mdDoc ''

          As alternative to ``settings``, you can provide whole configuration

          directly in the almost-YAML format of Knot DNS.

          You might want to utilize ``writeTextFile`` for this.

        '';

      };

      package = mkOption {

        type = types.package;

        default = pkgs.knot-dns;