[Fargate][ECS] [Feature Request]: Allow Fargate to run only 1 task per Availability Zone

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**Tell us about your request**
What do you want us to build?

Create some sort of Daemon Set option for Fargate in which only 1 Task per Availability Zone will be scheduled

**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?**
Currently there is no way to do this, and the ECS Scheduler will spread tasks evenly across AZs, and if one AZ goes down, a Task might end up on the same AZ as the other one.

**Are you currently working around this issue?**
The only work around is to set the Desired Count equal to the number of subnets in the service, but if one eventually goes down, a task will eventually be placed on the same subnet as another one already running

**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.)

This is a roadmap request for Fargate/ECS scheduling rather than a repository change, and the payload names no files, tests, or entry points. Start by researching the ECS Scheduler's Availability Zone placement behavior and the requested one-task-per-AZ constraint; done means Fargate reliably maintains at most one task in each Availability Zone, including after an AZ failure.

Written by the indexing model from the issue text.