nixos/samba: Allow package version setting

nixos/modules/services/network-filesystems/samba.nix

@@ -9,7 +9,7 @@ let
   logDir = "/var/log/samba";
   privateDir = "/var/samba/private";
 
-  inherit (pkgs) samba;
+  samba = cfg.package;
 
   setupScript =
     ''
@@ -90,6 +90,14 @@ in
         ";
       };
 
+      package = mkOption {
+        default = pkgs.samba;
+        example = pkgs.samba4;
+        description = ''
+          Defines which package should be used for the samba server.
+        '';
+      };
+
       syncPasswordsByPam = mkOption {
         default = false;
         description = "