Stub ECS StopTask
Unlike the real endpoint which allows for some shutdown time to respond
to SIGTERM, this immediately switches the lastStatus to STOPPED.
The real ECS eventaully garbage collects stopped tasks so they'll no
longer appear in things like ListTasks; our stubbed ECS does not.
Test Plan: unit
Reviewers: alangenfeld, dgibson, johann
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8403