nixos/bind: set type of zones.*.extraConfig to lines

allowing multiple definitions of that option to be merged, because:
- their order should be irrelevant
- it might make sense to declare multiples of them at different locations

nixos/modules/services/networking/bind.nix

@@ -45,7 +45,7 @@ let
         default = [ "any" ];
       };
       extraConfig = lib.mkOption {
-        type = lib.types.str;
+        type = lib.types.lines;
         description = "Extra zone config to be appended at the end of the zone section.";
         default = "";
       };