nixos/steam: add option extraPackages

nixos/modules/programs/steam.nix

@@ -55,6 +55,7 @@ in {
             then [ package ] ++ extraPackages
             else [ package32 ] ++ extraPackages32;
         in prevLibs ++ additionalLibs;
+        extraPkgs = p: (cfg.extraPackages ++ lib.optionals (prev ? extraPkgs) (prev.extraPkgs p));
       } // lib.optionalAttrs (cfg.gamescopeSession.enable && gamescopeCfg.capSysNice)
       {
         buildFHSEnv = pkgs.buildFHSEnv.override {
@@ -71,6 +72,19 @@ in {
       '';
     };
 
+    extraPackages = lib.mkOption {
+      type = lib.types.listOf lib.types.package;
+      default = [ ];
+      example = lib.literalExpression ''
+        with pkgs; [
+          gamescope
+        ]
+      '';
+      description = ''
+        Additional packages to add to the Steam environment.
+      '';
+    };
+
     extraCompatPackages = lib.mkOption {
       type = lib.types.listOf lib.types.package;
       default = [ ];