Both `list` and `status` with `--json=pretty|short` show the description.
It is fetched via D-Bus by org.freedesktop.network1.Manager.Describe
This change exposes the same data via Varlink API `io.systemd.Network.Describe`
and migrates networkctl list and status commands to use Varlink API.
Update GetStates IDL to use enums for link states.