feat: add custom time source injection for testing

[CybotTM]

Summary

Allow users to inject a custom time.Now() function for testing and special use cases.

Background

Reference: robfig/cron PR robfig#549

Currently, the scheduler always uses time.Now(). This makes testing time-dependent behavior difficult without actually waiting.

Proposed Solution

Add a functional option:

func WithClock(clock func() time.Time) Option {
    return func(c *Cron) {
        c.clock = clock
    }
}

Considerations

• WithLocation() covers most timezone use cases
• Adds API complexity
• Low demand from users
• Reconsider if user demand increases

Use Cases

• Unit testing scheduled job execution
• Simulating time jumps for DST testing
• Replaying historical schedules