* feat: added nexdns dnsapi
Adds a DNS-01 hook for NexDNS, an authoritative DNS service with a REST API.
dns_nexdns_add walks the label list to find the zone that owns the challenge
name and creates the TXT record in it. dns_nexdns_rm lists the TXT records at
that name, picks the one carrying exactly this challenge value and deletes it
by id, so a wildcard and its base domain do not remove each other's record.
A 429 is waited out and the request retried, in the shape dns_hetznercloud.sh
and dns_bunny.sh already use.
* dns_nexdns: cap the rate-limit wait, judge success by status, add the tracking issue
