zerotier: Migrate from clanModule to clanServices

This migrates zerotier from clanModules to clanServices.
This is a graceful migration and does not yet disables any of the
clanModule functionality.

clanServices/zerotier/flake-module.nix

@@ -0,0 +1,9 @@
+{
+  lib,
+  ...
+}:
+{
+  clan.modules = {
+    zerotier = lib.modules.importApply ./default.nix { };
+  };
+}