Add mlf-dns-namecheap plugin

Namecheap's XML-over-GET legacy API. Every call hits xml.response with ApiUser/ApiKey/UserName/ClientIp params; production access requires whitelisting the caller's IP in Namecheap's admin panel (the plugin surfaces the common "IP is not in the whitelist" response cleanly as IpNotWhitelisted rather than an opaque HTTP 200 with an error body). setHosts replaces every DNS record on the zone, so upsert/delete round-trip through getHosts, modify-in-memory, setHosts. Zone lookup walks parent domains against domains.getList. Options schema takes api_user, api_key, user_name (defaults to api_user), and a required non-secret client_ip so the user can set it correctly per environment.