feat(core): Schedule class

[kaizencc]

My latest attempt at schedule unification across modules.

The following modules use a version of schedule, which this PR aims to unify:

• aws-scheduler-alpha
• aws-events
• aws-application-autoscaling
• aws-autoscaling
• aws-synthetics-alpha
• aws-backup

The idea is to have a single source of truth, core.Schedule that is exposed and meant to be extended by modules that use a schedule. This is to avoid breaking changes -- every module that currently exports a schedule class continues to do so. Each module can customize their schedule class to its liking, for example, whether or not to support schedule.at or cronOptions.timeZone.

This PR will fix inconsistencies like:

• backup.scheduleExpression depending on events.Schedule, which is semi-deprecated by the Events team (they want people to use the Schedule class in aws-scheduler-alpha).
• aws-scheduler-alpha depending on events.Schedule as well.
• backup.scheduleExpression allowing schedule.rate(duration) to be specified (synth-time error) when we know that backup schedules only can be cron expressions.
• having to implement the new timeZone property in all instances of schedules
• avoids us from having to perform maintenance in multiple places like chore(aws-events): add warning when minute is not defined #19197
• timeZone property existing directly on a construct when it only pertains to cron expressions. This is an anomaly because we typically do not want construct-level properties to only be impactful depending on other properties. [See superseded PRs]

Challenges:

• subtle differences in expressions that are accepted. This is solved by core.Schedule only exposing protected APIs, which are then picked by the consuming modules to be exposed as public.
• subtle difference in cron expressions accepted. I do some magic in aws-autoscaling to get the cron expression returned there to be as expected.

Supersedes #27052 and #27012

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[rix0rrr]

Nice! I'm conditionally approving this to allow you to get on with things.

A potentially even nicer solution to get rid of the Schedule subclass proliferation (I didn't know it had gotten that bad) could be to just have the singular core.Schedule class, that holds on to its input and returns it unrendered, and everyone accepts that class as a parameter.

It's then each service's responsibility to render that data to a string as the service would accept it.

In other words: holding+rendering is now the responsibility of each service's Schedule class, but we could make core.Schedule do the holding, and autoscaling.ScalableTarget do the rendering.

I'll leave it to you to decide.

[mergify]

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 1efab47
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[saudansari]

Having timezone in CRON was useful for ECS scheduled tasks as well. Ever since this change, that feature is broken. I'd appreciate if timezone can be added to ecs.scaleOnSchedule