Add scheduler snippets for systemd timers

[amotl]

Is your feature request related to a problem?
We want to run wetterdienst as a cronjob. These days, systemd timers is the place to go more often than not.

Describe the solution you'd like
Use @maxhumber's hickory (thanks!) to quickly add scheduling to wetterdienst without efforts, still based on OS-level machineries like systemd for Linux and launchd for macOS.

Describe alternatives you've considered
Do that manually by reading through things like

• https://opensource.com/article/20/7/systemd-timers
• https://kofler.info/systemd-timer-als-cron-alternative/

Additional context
At Luftdatenpumpe, we had a blueprint for a cronjob at luftdaten-viewer.cron and respective documentation at Luftdaten-Viewer Cron Job. However, we want to get the same thing for systemd timers in order to provide users a concise guideline about how to setup wetterdienst running as a cronjob. When hickory can fulfill this job, it would be even better as we can pull it into wetterdienst's dependencies and make it schedule itself.

cc @wetterfrosch