[terraform] Create an alarm for when the identity ECS task stops

Summary:
This creates an alarm for when the identity ECS task stops, notifying error report subscribers.

Depends on D11854

Test Plan: Will not land until verified that after shut down of identity ecs task, email notification is sent

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11892