[FEATURE] Global DNS configuration

[cachebag]

pop-os/cosmic-settings#2087

NetworkManager exposes a writable GlobalDnsConfiguration property on /org/freedesktop/NetworkManager. nmrs has no support for reading or writing it.

this blocks COSMIC settings from offering a "change DNS servers globally" option — currently users can only set DNS per-connection.

[sage-s11]

I'd like to take this.

Plan: add GlobalDnsConfiguration on the NM proxy in dbus/main_nm.rs, a typed struct (searches / options / domains["*"].servers), and NetworkManager::global_dns_configuration + set_global_dns_configuration. Empty dict clears the global override.

Unit tests for the a{sv} round-trip; integration test in the Docker NM harness that sets, reads back, and restores. Following the same pattern as #547.

I'll open a PR in the next few days.