[Response Ops][Alerting] Refactor `ExecutionHandler` Pt 1

Rename ExecutionHandler to ActionScheduler.
Create the different scheduler classes (SummaryActionScheduler, SystemActionScheduler, PerAlertActionScheduler)
In this first stage, we'll split the for-loop currently inside ExecutionHandler.generateExecutables into the appropriate class and combine the returned executables from each scheduler class. For this stage, we'll keep the second for-loop that loops over the executables inside the ExecutionHandler/ActionScheduler class.

elastic / kibana