nixos/yggdrasil: ensure freeformType attrs are resolved

`lib.types.attrs` is too lenient, allowing for invalid yggdrasil
configuration to make it's way into the final config. Since this
configuration ends up in a JSON file, changing the `freeformType` to
use the type definition from `pkgs.formats.json` allows for the
configuration to resolve correctly.

nixos/modules/services/networking/yggdrasil.nix

@@ -13,7 +13,6 @@ let
     bool
     listOf
     str
-    attrs
     submodule
     ;
   cfg = config.services.yggdrasil;
@@ -69,7 +68,7 @@ in
 
       settings = mkOption {
         type = submodule {
-          freeformType = attrs;
+          freeformType = (pkgs.formats.json { }).type;
           options = {
             PrivateKeyPath = mkOption {
               type = nullOr path;