Add support for transient timers

[onitake]

systemd-run supports creating units that run on a schedule instead of immediately. This is implemented by creating a timer unit alongside the service that executes the command.

While creating persistent timers and services is certainly an option, they create manual cleanup overhead. It would be very useful to support such a feature in go-systemd, similar to systemd-run's --timer-property= option.

Similarly, systemd-run also supports creating transient socket and path units that launch the command service when triggered. These would also be useful.