sddm: allow extra config options

nixos/modules/services/x11/display-managers/sddm.nix

@@ -35,6 +35,8 @@ let
     SessionCommand=${dmcfg.session.script}
     SessionDir=${dmcfg.session.desktops}
     XauthPath=${pkgs.xorg.xauth}/bin/xauth
+
+    ${cfg.extraConfig}
   '';
 
 in
@@ -50,6 +52,19 @@ in
         '';
       };
 
+      extraConfig = mkOption {
+        type = types.str;
+        default = "";
+        example = ''
+          [Autologin]
+          User=john
+          Session=plasma.desktop
+        '';
+        description = ''
+          Extra lines appended to the configuration of SDDM.
+        '';
+      };
+
       theme = mkOption {
         type = types.str;
         default = "maui";