atomdns,libdns: add (minimal?) interface to add acme dns-01 challenge to zones #874

see e.g. https://github.com/libdns/luadns or maybe even simpler ones exist; where we just expose enough to add (and expire autom.?) an dns-01 acme challenge.

This could be all in memory. We might need to do it twice, for both servers I'm running in the cluster, but this should be doable...

this requires

• some https endpoint (or should we just use dyn. update; as this save me from running a webserver and getting TLS for that too)
• simple handler that only handles update for txt records
• forgo deletion, just expire after a day

wrap this in a libdns interface and compile caddy with support for it.

the libdns wrapper, must

• resolve the DNS endpoint
• send an update msg to all endpoints
• (optionally tsig sign it with a shared secret)

the libdns interface is:

// Interface guards
var (
	_ libdns.RecordGetter = (*Provider)(nil)
	_ libdns.RecordAppender = (*Provider)(nil)
	_ libdns.RecordSetter = (*Provider)(nil)
	_ libdns.RecordDeleter = (*Provider)(nil)
	_ libdns.ZoneLister = (*Provider)(nil)
)

Think we should get away with Getter, Setter and Lister, making Deleter a noop and Appender just call Setter.

We're gonna something to distribute the caddy data ourselves, see https://github.com/psviderski/uncloud/issues/31#issuecomment-3566008651

which is kinda of ugly, but also nice, as it forgoes all this dns dancing - which can also work, but looks more fragile