nixos/jupyter: make kernel options freeform JSON

nixos/modules/services/development/jupyter/default.nix

···
 
     kernels = mkOption {
       type = types.nullOr (types.attrsOf(types.submodule (import ./kernel-options.nix {
-        inherit lib;
+        inherit lib pkgs;
       })));
 
       default = null;

nixos/modules/services/development/jupyter/kernel-options.nix

···
 # Options that can be used for creating a jupyter kernel.
-{ lib }:
+{ lib, pkgs }:
 
 with lib;
 
 {
+  freeformType = (pkgs.formats.json { }).type;
+
   options = {
 
     displayName = mkOption {

nixos/modules/services/development/jupyterhub/default.nix

···
 
     kernels = mkOption {
       type = types.nullOr (types.attrsOf(types.submodule (import ../jupyter/kernel-options.nix {
-        inherit lib;
+        inherit lib pkgs;
       })));
 
       default = null;