batou_ext.cron.SystemdTimer: Allow to supply multiple onCalendar expressions

[tritum]

Systemd allows to supply timers with multiple values for OnCalendar. This will trigger the service every time any timer matches. The SystemdTimer class does, however, only supports supplying a single string value.

NixOS seems to be able to handle multiple timer units. Local testing suggests that supplying a list should be able to generate a valid systemd timer:

systemd.timers.test_timer = {
    # …
    timerConfig.OnCalendar = ["*-*-01 01:00:00" "*-*-2 02:00:00"];
}

# …

[Timer]
OnCalendar=*-*-01 01:00:00
OnCalendar=*-*-02 02:00:00

An alternative would be to create multiple timers/services doing the same thing. But as each timer
is marked as Persistent, my understanding is that such jobs could be triggered multiple times after a system failure.