[ECS] [request]: de-register from Cloud Map / R53 when instance is draining

**Tell us about your request**
I want ECS to better handle the state of DRAINING for tasks when it comes to using service discovery, in our case initiated by draining of EC2 instances

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We are running software using service discovery that needs to be gracefully drained, the same way that is handled with having a load balancer and draining, ie removing from load balancing traffic when draining.

The current working of ECS is that it does not remove the current task from Cloud map when it is put in draining which is the desired behavior so that we can drain the traffic from the instance

** What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.**
If we cannot gracefully drain the traffic from our task, the clients of that service will get an unnecessary high rate of connections that just die, possibly mid-request. Forcing uncessary resync and checking whether request was received on the target-end.

**Are you currently working around this issue?**
There is currently not any good work-around that we can find to this problem.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reading the ECS service-discovery behavior described in this issue, especially how tasks on draining EC2 instances are represented in Cloud Map and Route 53. Done means a task is de-registered from service discovery when its instance enters DRAINING, allowing existing traffic to drain without new connections; the issue names no repository files or tests.

Written by the indexing model from the issue text.