define selfprivacy.passthru option (type = types.submodule)

Stock NixOS passthru option cannot be defined in multiple places. But we
need to pass arbitrary parameters between SP modules.

selfprivacy-module.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ lib, pkgs, ... }:
 
 with lib;
 {
@@ -139,5 +139,20 @@ with lib;
         default = null;
       };
     };
+    ################
+    # passthrough  #
+    ################
+    passthru = mkOption {
+      type = types.submodule {
+        freeformType = (pkgs.formats.json { }).type;
+        options = { };
+      };
+      default = { };
+      visible = false;
+      description = ''
+        This attribute allows to share data between modules.
+        You can put whatever you want here.
+      '';
+    };
   };
 }