resolved: expose a new bus property that informs about the /etc/resolv.conf mode

resolved: expose a new bus property that informs about the /etc/resolv.conf mode

It can be one of "foreign", "missing", "stub", "static", "uplink",
depending on how /etc/resolv.conf is set up:

foreign → someone/something else manages /etc/resolv.conf,
    systemd-resolved is just the consumer

missing → /etc/resolv.conf is missing altogether

stub/static/uplink → the file is managed by resolved, with the
    well-known modes

Fixes: #17159
(cherry picked from commit 4261ab654c0df009b5a37a30593d58d47fec617f)