[Response Ops] Refactor alerting task runner - AlertingEventLog class

[ymao1]

Meta issue: #124206

As a step towards refactoring the task runner, we want to encapsulate all event log transactions that occur during task running into a class (AlertingEventLog) with the following public functions:

• initialize() - initializes the class with all the data required to populate the common metadata fields
• start() - sets the start time for the transaction (shared between execute-start and execute and writes out the execute-start event
• logActiveAlert()/logNewAlert()/logRecoveredAlert()/logTimeout()
• complete() - sets metrics data and any errors/warnings for rule run and writes out execute event

Ensure this class is thoroughly unit-tested.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)