Add seconds granularity in cron-trigger execute

Problem description
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Allow seconds interval for cron-trigger

Proposed change
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1) Replace croniter with apscheduler (Advanced Python Scheduler).
2) The new Scheduler will be BackgroundScheduler (A scheduler that runs in the background using a separate thread)
3) A new method will be added to triggers.py class, convert_pattern_to_numbers(patterns). This method will convert from the crontab pattern to sec/min/hour/day of month/month/day of the week in numbers

User Experience shouldn't change 5 fields cron expression will still be supported in addition to the new 6 fields format (sec/min/hour/day of month/month/day of the week)

Alternatives
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this is the only python library I found the support seconds in scheduled job

Use-cases
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monitoring an application (for example stack-check every 30 sec)