Allow creating wildcard domains in `baseDomains`

Some services need wildcard dns support so it makes sense to configure them in `baseDomains`. Wildcard domains work fine in `dns.Config.extraConfig.zones.<name>`, but not in `baseDomains` because it tries to create a zone file called `*.example.com`. I tested a fix with a local checkout of NixOS-DNS where I replace the `*` with `_wildcard` and that fixed the issue. But I'm not sure if that's the preferred solution.