Issue:
Currently, application autoscaling, autoscaling, and events have their own schedule class that are very similar. They all support a subset of these static methods: at(moment), rate(duration), expression(expression), and cron(options). Because each resource has different ways of specifying its schedule class, this PR is not as simple as moving the class to core and removing duplicates.

Solution:
This PR moves the repetitive logic to core as a BaseSchedule class, with all methods protected. It is called a Base class because other resources that have a need for a schedule should extend this and expose the necessary methods. This way, one BaseSchedule class can support multiple similar renditions of the schedule property.

Note:

• I did not add an explanation in the README because BaseSchedule should never need to be referenced by cdk users. This results in the PR Linter failure.
• The cron option in autoscaling follows the UNIX format with 5 units (source). However, application autoscaling and events follows a slightly different format with 6 units (source). To accommodate both formats, I added an optional unix parameter to createCron that if set, will return a UNIX formatted cron expression.

Closes #8979

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