Support configuring cron schedule dynamically with a block

[DanielHeath]

Implements #1050 (comment)

[bensheldon]

@DanielHeath I just pushed up some changes.

• Now uses the last in-memory time the job was scheduled or the time in the database. This is necessary because the next task is scheduled before the previous task is enqueued.
• I wish the database query could be removed altogether because it happens during the Rails initialization process and I've seen that cause autoloading deadlocks in the past... but for safety I wrapped it in a Rails.application.executor.
• Added a few more tests

[DanielHeath]

Anything else required for this to be merged?

[bensheldon]

Released 🎉 https://github.com/bensheldon/good_job/releases/tag/v3.18.0