[ECS] [request]: public IPs for awsvpc

**Tell us about your request**

Allow an awsvpc ENI to have a public IP address or elastic IP.

**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 want to have both the security features of awsvpc networking and public internet access.

I prefer not to have the complexity, cost, performance, or availability limitations of an NAT gateway.

**Are you currently working around this issue?**

Workarounds:

* Set up a NAT gatway
* Use Fargate
* Use host or bridge networking

**Additional context**

Fargate has had public IPs for awsvpc ENIs from day 1. Presumably it wouldn't be hard to add this for EC2-based containers.

This is an ECS roadmap request rather than a repository implementation task. Start by reviewing the awsvpc networking request, its listed workarounds, and the stated Fargate precedent. Done means EC2-based ECS awsvpc ENIs can receive a public or elastic IP without requiring the listed NAT gateway workaround.

Written by the indexing model from the issue text.