[Feature] Add interval trigger type for workflow schedules

[liang-wenjie]

Feature request

Add an interval-based trigger type for workflow schedules.

Motivation

Some workflows should run at a fixed interval instead of using a Cron expression. When executions overlap, the scheduler should continue calculating fire times from the configured interval. Users should also be able to configure a finite execution count or unlimited repetition.

Proposed behavior

• Support Cron and interval trigger types.
• Configure interval by hours, minutes, and seconds.
• Support a repeat count, where -1 means unlimited repetitions.
• Preview the next interval-based execution times.
• Respect the existing missed-fire policy.