[EKS] [request]: EKS managed node group support for ASG target group

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**Tell us about your request**
The ability to attach a load balancer to the ASG created by the EKS managed node group at cluster creation with cloudformation.

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We used to create an unmanaged node group with ASG and a classic load balancer in the same cloudformation stack. We used `!Ref` to attach the load balancer to the ASG using `TargetGroupARNs`. However, the configuration is not available in eks managed node group at cluster creation today.

**Are you currently working around this issue?**
We need to separate the creation of cluster and the load balancer into two stacks while they have the same lifecycle. Besides, we are not sure if this modification to ASG is allowed and supported since the ASG is managed by EKS.

Start by reviewing how EKS managed node groups are represented during CloudFormation cluster creation and how ASG TargetGroupARNs are handled for unmanaged node groups. Confirm whether attaching the load balancer target group is supported for EKS-managed ASGs; done means the target group can be configured in the same stack and lifecycle without requiring separate stacks.

Written by the indexing model from the issue text.