[ECS/Fargate+ALB] [request]: grace period configuration for unhealthy tasks

**Tell us about your request**
Add "grace period" configuration to configure delay between detecting unhealthy task and stopping it.

**Which service(s) is this request for?**
ECS/Fargate with ALB

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
If there is a problem with task and it becomes unhealthy, load balancer stops sending traffic to this task. It will resume after task becomes healthy again. This works as expected.

Problem is that ECS will immediately stop task once it is marked as unhealthy. I can slow this process by configuring draining period but once the task begins draining, it won't come back.

This is especially problematic with traffic spikes. Few tasks become overloaded and are marked as unhealthy – that is correct (I don’t want to route traffic to overloaded tasks). They would recover after few seconds but problem is that they are stopped and that makes the traffic spike worse because now even more requests is routed to other containers and it falls like domino.

**Are you currently working around this issue?**
It could be solved by not connecting ECS service to load balancer and handling target registration and task stopping from Lambda but it seems over overengineered.

We are currently rate limiting requests in containers and dropping those over limit. It would be far more practical to mark container as unhealthy to stop it from receiving more traffic.

**Additional context**
Might be relevant to #289 and #251