[ECS] [request]: Allow termination policies for services.

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**Tell us about your request**
It doesn't seem like there's any way to control which containers are shut down during a scale in. #125 touches on this a bit, but manually protecting a task from scale-in isn't exactly what I'm looking for. Ideally it would be great if we could get the equivalent of the termination policies that EC2 Auto-scale groups use,


**Which service(s) is this request for?**
This could be Fargate, ECS,

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
My specific problem is that I'd like to be able to tell the ECS service to always terminate the oldest container during a scale-in. We're running code that registers these containers with an external service and in cases where multiple containers try to register the most recent takes precedence. When ECS scales in the most recent container instead of the older ones it causes interruptions in the service.

**Are you currently working around this issue?**
None that I've been able to find yet.

No repository files, tests, or entry points are identified. Start by reviewing the ECS scale-in behavior described here and the related context in issue #125; done means reaching an agreed termination-policy design that selects the intended container reliably without service interruptions.

Written by the indexing model from the issue text.