The PR implements the following objects + tests for
io.systemd.Unit.List:
* PathContext
* PathRuntime
* ScopeContext
* ScopeRuntime
* SwapContext
* SwapRuntime
* TimerContext
* TimerRuntime
It's a continuation of the following PRs:
* https://github.com/systemd/systemd/pull/37432
* https://github.com/systemd/systemd/pull/37646
* https://github.com/systemd/systemd/pull/38032
* https://github.com/systemd/systemd/pull/38212
* https://github.com/systemd/systemd/pull/39391