new: usr: support for zone-specific plugins

Query plugins can now be configured at the `zone` level, as well as globally or at the `view` level. A plugin's hooks are then called only while that specific zone's database is being used to answer a query.

This simplifies the implementation of plugins that are only needed for specific namespaces for which the server is authoritative. It can also enable quicker responses, since plugins will only be called when they are needed.

Closes #5356

Merge branch 'colin/zone-plugins' into 'main'

See merge request isc-projects/bind9!10483