Ensuring that the tz database is up-to-date

[irvinlim]

Furiko currently uses the tz database bundled with the Go runtime to provide timezone support: https://furiko.io/guide/execution/jobconfig/scheduling/#crontimezone

While this avoids dependencies on the external environment in which Furiko is run, it tightly couples the recency of the data to the version being deployed.

We will need to investigate ways to get around this. For example, we can periodically update the tz database in the background, falling back to the bundled database if not available (e.g. deployed in airgapped environments without Internet, etc). We also need to add configuration options for the administrator to decide the following:

• Whether to use bundled tz database or not (enabled by default, mostly for convenience)
• Whether and where to load an external tz database from file
  • Covers use cases of air-gapped environments, and when cluster administrator wants to explicitly manage the tz database to ensure it is up-to-date

[irvinlim]

FWIW, Kubernetes 1.24 introduces timezone support for CronJobs. The KEP is as follows: https://github.com/kubernetes/enhancements/tree/master/keps/sig-apps/3140-TimeZone-support-in-CronJob

However, the KEP does not present a solution to fix this problem. This is also mentioned in the discussion for the PR which implements the KEP: kubernetes/kubernetes#108032 (review)