[ECS] [networking]: Allow public IPv4 addresses to be assigned to the ENIs of EC2 tasks

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**Tell us about your request**
I'd like to be able to assign a public IPv4 address to each container running on an ec2 instance.

**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?**
I have a docker container doing some custom streaming/networking that needs certain weird ports exposed to communicate over (i.e., so I can't use elastic beanstalk), and I'd like to be able to run multiple copies of this container per EC2 instance. I would just use dynamic port forwarding, but the protocol I'm using would need a number of changes to be able to support communicating through arbitrary ports (I'd also need to be able to see the precise host ports being forwarded to each container port the moment the task starts running, and I'm unsure how to do that -- if there's an easy way to get that from the describetask or describecontainerinstances API that would also help).

**Are you currently working around this issue?**
Right now I'm using fixed port forwarding and running one task per EC2 instance.

**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.)

The issue names ECS tasks, EC2 ENIs, Docker containers, and the DescribeTask and DescribeContainerInstances APIs, but no repository files or tests. Start by clarifying the required ECS networking behavior and how public IPv4 assignment interacts with task placement and ports. Done means an agreed implementation scope and acceptance criteria for assigning addresses to multiple tasks per EC2 instance.

Written by the indexing model from the issue text.