lightdm-greeters service: add extraConfig option (#24135)

nixos/modules/services/x11/display-managers/lightdm-greeters/gtk.nix

@@ -45,6 +45,7 @@ let
     theme-name = ${cfg.theme.name}
     icon-theme-name = ${cfg.iconTheme.name}
     background = ${ldmcfg.background}
+    ${cfg.extraConfig}
     '';
 
 in
@@ -103,6 +104,15 @@ in
 
       };
 
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Extra configuration that should be put in the lightdm-gtk-greeter.conf
+          configuration file.
+        '';
+      };
+
     };
 
   };