sysupdated: Split Update() into Acquire() and Install() D-Bus methods (#40628)

These mirror the `sysupdate acquire` and `sysupdate install` verbs,
which are themselves a split of `sysupdate update` into downloading and
installing stages.

I have dropped the existing `Update()` method in the final commit, as
per the suggestion on
https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/2004#note_2145880.
If the D-Bus API for `sysupdated` is considered stable now, though, I
can drop that commit.

CC @AdrianVovk

Fixes https://github.com/systemd/systemd/issues/34814

diff --cc test/units/TEST-72-SYSUPDATE.sh
Simple merge