Add ability to manually trigger a scheduled task to validate the task configuration

[ElanHasson]

Describe the feature

The proposal here is to add a ManualTrigger() method to the ScheduledTaskGrain which triggers the task, ignoring the task.IsEnabled and task.NextRunAt properties.

ManualTrigger() should return the ScheduledTaskTriggerHistory object instead of adding the result to the Task's Trigger History.

This is a diagnostic endpoint and should be protected via the auth mechanism.