Adding task-level alert support

[David Yu]

Hi, in the alert_on_failure function documentation it says the function could work for both flow and tasks, but the current repo only works for flow-level alerts. Is task-level support currently on the timeline?

[Khuyen Tran]

I'm not sure where you saw that the decorator works for both flows and tasks, but currently, this decorator doesn't work with tasks. It is difficult to make this decorator work with task.submit and would require Prefect code base to change so this might take a couple of weeks to be implemented