refactor!: Refactor cronjob controller to reduce duplication

[Mrtenz]

The cronjob controller contained a lot of essentially duplicated logic after the addition of background events. To resolve this, I've refactored the cronjob controller to treat cronjobs as recurring background events. This means:

• Cronjobs and background events are now (re)scheduled and executed using the same logic.
• Cronjobs are stored in state just like background events.
  • I removed the lastRun property in favour of a date property. We can now simply check if the current date is past the scheduled date to know if a cronjob should be executed immediately.
  • This simplifies the logic for rescheduling cronjobs significantly.
• Cronjobs can now use ISO 8601 durations as well as the usual cron expression.
• The logic for getting all cronjobs from all Snaps was removed, since the full cronjobs are stored in state now.
  • This works fine for new Snaps, but I need to consider how to handle existing Snaps at the time of the cronjob controller update, either with a migration or by re-adding some of this logic.

In addition to these changes, I've made some smaller changes to the controller like replacing TypeScript's private modifier with true hash private functions.

Breaking changes

• The jobs state property was removed, cronjobs are now stored in the events property as well.
• CronjobController:schedule now expects a schedule field instead of date.
• Some actions were renamed to remove the BackgroundEvent suffix.
  • CronjobController:scheduleBackgroundEvent -> CronjobController:schedule
  • CronjobController:cancelBackgroundEvent -> CronjobController:cancel.
  • CronjobController:getBackgroundEvents -> CronjobController:get.

[Mrtenz]

The logic for scheduling was previously done in snaps-rpc-methods, but handling this from the cronjob controller is easier to reduce duplication.

[FrederikBolding]

We need to find a way to test both formats 😓

[Mrtenz]

Likely easiest with a new example Snap. Just changed it here for local testing purposes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.18%. Comparing base (8854c9f) to head (dfb3b78).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3421      +/-   ##
==========================================
+ Coverage   98.16%   98.18%   +0.01%     
==========================================
  Files         402      399       -3     
  Lines       11141    11127      -14     
  Branches     1750     1750              
==========================================
- Hits        10937    10925      -12     
+ Misses        204      202       -2     

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[FrederikBolding]

@Mrtenz Can you add a note about the breaking changes in the description please?