CronWorkflow referencing custom calendars (ie with public holidays defined)

define a custom calendar that has some public holidays excluded (maybe in some configmap)
then set a cronwf to run on the days defined in the custom calendar

The issue proposes custom calendars with public holidays, possibly in a ConfigMap, and using them from a CronWorkflow. No files, tests, or entry points are named; first clarify the calendar format, configuration mechanism, and scheduling semantics before identifying the implementation and validation work.

Written by the indexing model from the issue text.