[ECS/Fargate] [request]: Support blue/green deployment for an ECS service with multiple target groups

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**Tell us about your request**
Very similar to #760 but we are not as able to proxy as they were.
Blue/Green Fargate deployments with traffic control that can handle multiple target groups per service.

**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?**
We currently run an application in Fargate that receives traffic on 5 different ports, 2 public and 3 privte, going through NLBs. It would make autoscaling and cost optimisation much more effective if we could receive that traffic on a small group of base containers, and scale up when needed, instead of the current situation where we will need a minimum of 5 tasks * autoscaling required for HA operations.

**Are you currently working around this issue?**
We have 5 services with 2 tasks in each service that only handles 1 of the 5 network ports. This makes scaling very inefficient.

**Additional context**
This feature could take our application from the current minimum of 9 containers to a minimum of 3.

Start by reading the related request in issue #760 and reviewing the described ECS/Fargate setup: NLB traffic across five ports and multiple target groups. Done means blue/green deployment support for an ECS service with multiple target groups, while addressing the stated scaling, high-availability, and separate-service constraints.

Written by the indexing model from the issue text.