Revisit systemd notifier

[dominickpastore]

The systemd notifier needs to be revisited for two reasons:

1. It's worth considering a different DBus library. The current libraries are unwieldy and have ugly dependencies. Two promising options are dasbus and dbus-next. dasbus is actively maintained and is designed for Python 3 but still has some ugly dependencies (specifically, PyGObject). dbus-next has a seemingly well-designed API and no crazy dependencies, but currently (Jan 1 2023) has not had any new releases in 18 months (though has had new commit activity only 3 months ago). The other options at https://wiki.python.org/moin/DbusExamples are either unmaintained or require libraries we aren't using.

2. When originally implemented (Q3 2021, 15-18 months ago), the DBus interface for systemd-networkd was not well documented, and we had to rely a lot on introspection. See if that has changed.