Formal timezone support in SpannerAutoscalerSchedule

[apstndb]

What you want to add

I think there are two options.

1. Document the current CRON_TZ support which is handled by github.com/robfig/cron/v3.
2. Implement timeZone field as like Kubernetes CronJob
   • https://github.com/kubernetes/enhancements/blob/master/keps/sig-apps/3140-TimeZone-support-in-CronJob/README.md
   • https://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/#time-zones

Why this is needed

Internal users of Mercari and many external users want to define schedules in local timezone.
For example, there are cases where the time zone is important to represent the beginning of the month in the calendar.
e.g. YYYYYY-MM-01 00:00:00 JST can be any of YYYY-${MM-1}-{28,29,30,31} 15:00:00 UTC so it is hard to be written in UTC.

• We had recognized needs of TZ support: Support schedule configuration like CronJob #30 (comment)