Fix: Cron Trigger Plan-time Validation; Cron Trigger Updates; Cron Trigger Testing

[korenyoni]

What

• Add Cron Trigger plan-time validation
• Fix Cron Trigger updating (force replacement of resource on any change, because there is no update verb in the cron trigger API)
• Improve Cron Trigger Testing
• Misc: remove use of "Hermes" service name in Provider logs. While accurate in the API docs, this may cause unnecessary confusion to users of the Terraform provider.

Why

• Updating a Cron Trigger results in duplicates
• No plan-time validation on Cron Triggers

Notes

• Closes Cron Trigger Duplicated When You Try to Modify it #99
• Closes Improve Cron Trigger Validation Error Message #98
• Adding a minor label because now we have plan-validation, which can potentially break existing cron trigger configurations in Terraform)