[Response Ops] Refactor alerting task runner - single function for scheduling actions

[ymao1]

Blocked on #131547

Meta issue: #124206

As a step towards refactoring the task runner, we want to combine the logic inside

await Promise.all(
        alertsWithExecutableActions.map(
          ([alertId, alert]: [string, Alert<InstanceState, InstanceContext>]) =>
            this.executeAlert(alertId, alert, executionHandler, ruleRunMetricsStore)
        )
      );

await scheduleActionsForRecoveredAlerts<
        InstanceState,
        InstanceContext,
        RecoveryActionGroupId
      >({

into a single library function (scheduleActionsForAlerts) with a full set of unit tests.

[elasticmachine]

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