Add support for network zone DNS NOTIFY

[gbilic]

Hi all,
It would be cool to have a resolution available as soon as possible after the creation of new instances via dns server notification.
Also, currently, AXFR requests provide the zone with a serial number equal to the current timestamp, which triggers a systematic update on the slave servers.

[stgraber]

We may be able to have some events trigger a notify event for affected zones, though that may be a bit tricky to figure out in some cases.

For the serial, we don't have a way to change that as Incus generates the zone on demand and so has no way to actually version it.

[gbilic]

Another good way would be to send dynamic dns updates to authoritative servers.