Cron based promotions

[PashCracken]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

Add support for stage promotion based on a cron expression. Automatically promote the last freight if previous stage is healthy.

Motivation

In our current CD pipelines that do not use Kargo we typically have promotion to production run on a schedule. E.g every morning Monday-Thursday. When the sync windows in different production environments after this the changes are synched. This change would take Kargo closer to replace our current pipelines

Suggested Implementation

No. Do not know the code base good enough yet

[hiddeco]

Potential duplicate of #1328

[krancour]

Potential duplicate of #1328

I think they are only very slightly different, and probably should be addressed together.

We could probably close this if we just work this detail into the #1328 thread.