[EKS [request]: CNI custom networking to support more than one subnet.

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**Tell us about your request**
Customers using [VPC CNI custom networking](https://docs.aws.amazon.com/eks/latest/userguide/cni-custom-network.html) can create ENIConfig with **one** subnet per AZ. This request is to allow more than one subnet per AZ for ENIConfig. Referring to the original requested from [amazon-vpc-cni-k8s](https://github.com/aws/amazon-vpc-cni-k8s/issues/1207) for additional inputs.

With this feature, customers can start with a smaller subnet/CIDR block first with ENIConfig and easily expand by adding more subnets in the future to have more IPs available for pods when needed.

Example scenario:
The expectation is that start with a single subnet for pod IPs (e.g., CUST_SNET1=100.64.0.0/22 which allows ~1000 pods for a subnet within a single AZ1). Run cluster for a while. Let's suppose, it runs out of pod IP capacity one day. Add another subnet (e.g., CUST_SNET2=100.65.0.0/22 for additional ~1000 pods to the same AZ1). Update ENIConfig for the AZ1 with two subnets now. Same ENIConfig, then pick up IPs (or delegated prefix) from these two subnets. Please note that second CIDR for the subnet CUST_SNET2=100.65.0.0/22 is not allocated to the ENIConfig on day-1, rather it will be added later if needed. It allows continuing expansion by adding more CIDR/subnets as needed without developing a complex logic of multiple ENIConfigs per subnets and automating individual node annotations as the new node join the EKS cluster.

This feature would also help when it is hard for customers to allocate a large contiguous CIDR block vs adding multiple smaller non-contiguous CIDR blocks to the ENIConfig.

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Start with the linked EKS CNI custom networking documentation and the ENIConfig example in this issue, then review the related amazon-vpc-cni-k8s request for implementation context. Done means an ENIConfig can accept multiple subnets in one availability zone and use them for pod IP allocation without requiring multiple ENIConfigs or node annotations.

Written by the indexing model from the issue text.