Simplify programmatic scheduling of cron tasks with time zone

[vpavic]

This commit adds overloaded addCronTask to ScheduledTaskRegistrar that allows simpler scheduling of cron tasks with non-default time zone.

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In practical terms, currently we have to do this:

@Override
public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
    taskRegistrar.addCronTask(new CronTask(() -> {}, new CronTrigger("* * * * * ?", ZoneId.of("Europe/London"))));
}

While the changes from this PR allow us to reduce it to:

@Override
public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
    taskRegistrar.addCronTask(() -> {}, "* * * * * ?", ZoneId.of("Europe/London"));
}