[ECS] [request]: Fargate spot capacity rebalancing

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**Tell us about your request**
Proactive provisioning of new tasks upon termination signal

**Which service(s) is this request for?**
Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I want to minimize the interruption of capacity when a Fargate Spot instance is about to be terminated by having ECS attempt to provision a new instance in another AZ at the time of the notification, rather than waiting until after the task is deregistered from the target group. Basically, the Fargate equivalent of https://docs.aws.amazon.com/autoscaling/ec2/userguide/capacity-rebalance.html

So, if running 20 tasks, then go up to 21 tasks and down to 20. As opposed to 20 -> 19 -> 20.

Start with the requested Fargate behavior and compare it with the linked EC2 Capacity Rebalance documentation. The payload names no repository files, entry points, or tests; done means provisioning a replacement task before the Spot task is deregistered, maintaining 20 tasks without the 20-to-19-to-20 gap.

Written by the indexing model from the issue text.