nixos/user-groups: add a toggle for user accounts

Microscopic change that allows users to toggle user accounts, per user, conditionally.

nixos/modules/config/users-groups.nix

@@ -90,6 +90,16 @@ let
 
     options = {
 
+      enable = mkOption {
+        type = types.bool;
+        default = true;
+        example = false;
+        description = ''
+          If set to false, the user account will not be created. This is useful for when you wish to conditionally
+          disable user accounts.
+        '';
+      };
+
       name = mkOption {
         type = types.passwdEntry types.str;
         apply = x: assert (stringLength x < 32 || abort "Username '${x}' is longer than 31 characters which is not allowed!"); x;
@@ -394,7 +404,7 @@ let
       };
     };
 
-    config = mkMerge
+    config = mkIf config.enable (mkMerge
       [ { name = mkDefault name;
           shell = mkIf config.useDefaultShell (mkDefault cfg.defaultUserShell);
         }
@@ -417,7 +427,7 @@ let
         (mkIf (config.isNormalUser && config.subUidRanges == [] && config.subGidRanges == []) {
           autoSubUidGidRange = mkDefault true;
         })
-      ];
+      ]);
 
   };