feat: Add option to calculate intervals from job completion time for interval-based scheduling (fixes #565)

[iyashjayesh]

Summary

Adds a new job option WithIntervalFromCompletion() that changes how the next run time is calculated for interval-based jobs. Instead of calculating from the scheduled start time (default), jobs can now calculate from their completion time, ensuring consistent rest periods between executions.

Resolves #565
Related to discussion #564

Problem

Currently, jobs scheduled with DurationJob calculate their next run time from the scheduled start time, not from when they complete. This causes inconsistent rest periods when execution times vary.

Usage Example

s, _ := gocron.NewScheduler()

s.NewJob(
    gocron.DurationJob(5*time.Minute),
    gocron.NewTask(func() {
          // job task
    }),
    gocron.WithIntervalFromCompletion(), 
)

s.Start()

Did you document any new/modified functionality?

• Unit tests added and passing
• Existing tests still passing
• Documentation updated (README, godoc, examples)
• No breaking changes
• Issue [FEATURE] - Interval job to optionally include job duration #565 resolved

[JohnRoesler]

Fantastic work here - I appreciate the thorough testing and documentation 👍