blackout time support

# Summary

allow configuring ALL CronWorkflows (or even normal workflows) with a time range that they CANNOT start inside

# Use Cases

My target DB that my workflows write data to has a weekly outage at a pre-defined 30 minute block

Start by tracing the CronWorkflow and normal workflow scheduling entry points to understand where start eligibility is decided. The issue does not name files or tests; done would require an agreed configuration model for blackout ranges and verification that workflows cannot start during the configured outage window.

Written by the indexing model from the issue text.