resolvectl: re-implement normal status output using varlink (#40529)

resolvectl: re-implement normal status output using varlink (#40529)

Currently, `resolvectl status` (and other status commands like
`resolvectl dns $iface`) uses varlink when asked to print JSON, but uses
dbus otherwise. This PR re-implements the regular status output to use
varlink instead of dbus.

This both simplifies the `resolvectl status` code, and is an incremental
step in replacing dbus usage with varlink throughout `resolvectl`.

The first several commits move `dns-configuration.[ch]` from
`src/network/wait-online` to `src/shared`, and complete the JSON
dispatching so that all fields are available in a concrete type. The
next few commits re-implement global, link, and delegate status output.
Finally, the last commits consolidate and simplify the status logic
since varlink and the `DNSConfiguration` type can be used in all cases.