The PR implements the following objects + tests for
`io.systemd.Unit.List`:
- `KillContext`
- `AutomountContext`
- `AutomountRuntime`
- `MountContext`
- `MountRuntime`
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