Batch SQL: Support Cron syntax for polling schedule

[willdonnelly]

Currently our batch SQL connectors support two forms of schedule descriptions: Go duration strings like "5h" or "30m" or a special once-per-day schedule like "daily at 11:23Z". There are plenty of other schedules users might desire which cannot currently be expressed, mostly weekly, monthly, or bimonthly patterns.

If we supported cron expressions we could handle all of those and most other patterns people are likely to want, for instance:

• Once a week at 1:23AM on Thursdays: 1 23 * * 4
• Once a month at 5:00AM on the 1st: 0 5 1 * *
• Twice a month at 5:00Am on the 1st and 15th: 0 5 1,15 * *

The actual work here is basically:

1. Finding and vetting an appropriate cron expressions package which we can use.
2. Deciding how to represent cron expressions as a freeform string input in the schedule property, for instance "cron(1 23 * * 4)" or "cron when 1 23 * * 4" or something.
3. Detecting that pattern in schedule.Parse() and delegating to the cron expressions package.
4. Adding cron expressions to the schedule validation regex in each batch connector.