Have a cron task api schedule cleaner

[brendanheywood]

We have lots of tasks that we want to ramp down in non prod. At the moment we are doing this with a post sql cleaner but it's cumbersome.

There is quite a lot of overlap with this tracker to force the schedule in config.php which might be a better solution:

https://tracker.moodle.org/browse/MDL-65843

We are already ramping down cron itself to be less frequent at the OS level eg 1 min -> 5 mins but we additionally want to

• [ ] choose specific tasks and change their timing
• [ ] it would be nicer to have rulesets ie 'objectfs*' => midnight so we don't have to list every task
• [ ] disable some tasks