Add function for getting "ready" cron jobs with preflight filter for hijacking.

[peterwilsoncc]

Add the function wp_get_ready_cron_jobs() to return jobs ready to be run (with a timestamp in the past) with a preflight filter to allow plugins to use a custom cron storage mechanism.

This is to round out the functionality of #32656 to allow custom cron storage while making use of Core's system for spawning jobs.

[pento]

This is fine to commit to finish off #32656.

[peterwilsoncc]

45797.2.diff​ adds @since tags to the docblocks following Pento's code review and commit approval.

[peterwilsoncc]

In 44483:

Cron API: Add function and filter to return ready cron jobs.

Add the function wp_get_ready_cron_jobs() to return a modified version of the cron array limited to jobs ready to be run, ie with a timestamp of time() or earlier.

The new function includes the filter pre_get_ready_cron_jobs to allow for custom cron storage systems. This rounds out the functionality added in #32656.

Props Pento for code review.
Fixes #45797.

[desrosj]

This was detailed in the following dev note: ​https://make.wordpress.org/core/2019/01/23/cron-api-changes-in-wordpress-5-1/