nixos/misc/nixpkgs: add setting nixpkgs.config.allowUnfreePackages = ["list" "of "packages"]

Inspired by https://github.com/NixOS/nixpkgs/issues/197325 this adds a
new option nixpkgs.allowUnfreePackages, which merges additively and can
thus be defined in multiple modules close to where the unfree package is
installed.

I would have liked ot name this option
nixpkgs.config.allowUnfreePackages, to define it closer to where the
allowUnfree and allowUnfreePredicate are defined, but I didn't see how
this could be achived. I would welcome some guidance on how to do this.

nixos/modules/misc/nixpkgs.nix

@@ -20,6 +20,9 @@ let
       rhs = optCall rhs_ { inherit pkgs; };
     in
     lib.recursiveUpdate lhs rhs
+    // lib.optionalAttrs (lhs ? allowUnfreePackages) {
+      allowUnfreePackages = lhs.allowUnfreePackages ++ (lib.attrByPath [ "allowUnfreePackages" ] [ ] rhs);
+    }
     // lib.optionalAttrs (lhs ? packageOverrides) {
       packageOverrides =
         pkgs:

pkgs/stdenv/generic/check-meta.nix

@@ -158,7 +158,10 @@ let
   #   allowUnfree = false;
   #   allowUnfreePredicate = (x: pkgs.lib.hasPrefix "vscode" x.name);
   # }
-  allowUnfreePredicate = config.allowUnfreePredicate or (x: false);
+  # Defaults to allow all names defined in config.allowUnfreePackages
+  allowUnfreePredicate =
+    config.allowUnfreePredicate
+      or (pkg: builtins.elem (lib.getName pkg) config.allowUnfreePackages or [ ]);
 
   # Check whether unfree packages are allowed and if not, whether the
   # package has an unfree license and is not explicitly allowed by the

pkgs/top-level/config.nix

@@ -226,6 +226,23 @@ let
       '';
     };
 
+    allowUnfreePackages = mkOption {
+      type = with lib.types; listOf str;
+      default = [ ];
+      example = [ "ut1999" ];
+      description = ''
+        Allows specific unfree packages to be used.
+
+        This option composes with `nixpkgs.config.allowUnfreePredicate` by also allowing the listed package names.
+
+        Unlike `nixpkgs.config.allowUnfreePredicate`, this option merges additively, similar to `environment.systemPackages`.
+        This enables defining allowed unfree packages in multiple modules, close to where they are used.
+
+        This avoids the need to centralize all unfree package declarations or globally enable unfree packages via
+        `nixpkgs.config.allowUnfree = true`.
+      '';
+    };
+
     allowBroken = mkOption {
       type = types.bool;
       default = false;