[ECS] [request]: auto assign public IP with EC2 launch type

**Tell us about your request**
Please add support to auto assign public IP with EC2 mode and awsvpc network mode.

**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 trying to deploy a publicly accessible ECS service with EC2 mode and awsvpc network mode, however, when we ENABLE assign public IP in the network configuration of the cloudformation template, the error messages says it's not supported.

**Are you currently working around this issue?**
Currently don't have a workaround yet, although might consider Fargate.

Start by reviewing the ECS request for EC2 launch type with awsvpc network mode and the related CloudFormation network configuration. The work is complete when assigning a public IP is supported for this configuration without the reported unsupported error.

Written by the indexing model from the issue text.