[FEATURE REQUEST] can add opt for cron to set second

[kissIce]

Is your feature request related to a problem? Please describe. yes, my project want to set cron like The 50th second of every minute，but canot to finish it. because asynq no opt to set cron opt. it only can set min crontab express is minute，only a fixed interval second can be set。 not flexible

Describe the solution you'd like i want asynq can add opt or func to set cron opt, such as asynq have one func to set cron.WithSeconds() cron.WithParser()