feat: specify local-only domains

[tnyeanderson]

At the moment, it doesn't seem possible to specify that a domain (and its subdomains) should only ever be resolved locally.

There is private_networks which works for in the opposite direction (ensures local IP reverse lookups are not recursed upstream). There is also local_domain_name which is unfortunately a DHCP setting and seems to have no effect on this behavior (at least if DHCP is disabled as it is in my setup).

This PR adds the ability to ensure that certain domains are treated as local-only and are never sent to any upstream servers for resolution. Essentially, it allows you to manage DNS for a local domain through AdGuardHome while ensuring that requests for non-existent subdomains aren't leaked to an upstream resolver.