new: usr: add support for synthetic records

Add a query plugin which, in "reverse" mode, enables the server to build a synthesized response to a PTR query when the PTR record requested is not found in the zone.

The dynamically-built name is constructed from a static prefix (passed as a plugin parameter), the IP address (extracted from the query name) and a suffix (also passed as a plugin parameter).  An `allow-synth` address-match list can be used to limit the network addresses for which the plugin may generate responses.

The plugin can also be used in "forward" mode, to build synthesized A/AAAA records from names using the same format as the dynamically-built PTR names. The same parameters are used: the plugin will react and answer a query if the name matches the configured prefix and origin, and encodes an IP address that is within `allow-synth`.

Closes #1586

Merge branch '1586-synthreverse' into 'main'

See merge request isc-projects/bind9!10348