[ECS,Fargate] [request]: Attach specific ENIs

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

Using Cloudformation when creating an ECS service for Fargate I'd like to be able to provide a list of Elastic Network Interfaces (ENI) for the Network Configuration such that the Fargate ran container will have consistent IPs.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
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.**

We have a 3rd party server that we connect to over an AWS managed VPN. This 3rd party service requires that the client making the request (i.e. the fargate container) be a whitelisted IP in their Access Control List (ACL) and cannot whitelist multiple /28.

**Are you currently working around this issue?
How are you currently solving this problem?**

Creating dummy ENIs in each subnet to "reserve" non-whitelisted IPs such that the Fargate container will receive one of the "available" IPs that is a whitelisted IP. The other option is to not use Fargate and use traditional ECS whereby we attach ENIs to the managed EC2 instances.

Start with the ECS/Fargate service Network Configuration and its CloudFormation representation, focusing on how ENIs are selected for a service. Done means a service can receive a specified list of ENIs so its Fargate tasks use consistent, whitelisted IP addresses.

Written by the indexing model from the issue text.