resolve: add DumpDNSConfiguration to varlink API

resolve: add DumpDNSConfiguration to varlink API

Add io.systemd.Resolve.DumpDNSConfiguration. This provides the same
information as io.systemd.Resolve.Monitor.SubscribeDNSConfiguration,
but just returns the configuration once without the subscription logic.

In order to use the same definitions for DNSConfiguration et al. between
both interfaces, move the definitions to io.systemd.Resolve, and include
them in io.systemd.Resolve.Monitor.

This will be used to implement --json for resolvectl status.