[EKS] [request]: Simplify CNI custom networking

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**Tell us about your request**
Simplify and remove certain steps required to use custom networking with VPC CNI plugin.

**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?**
[Custom networking](https://docs.aws.amazon.com/eks/latest/userguide/cni-custom-network.html) is a feature that allows you to run pods using separate subnets and security groups from worker nodes, however multiple setup steps are required:
- Setting up secondary VPC CIDR blocks can be time consuming, and requires a long string of EC2 API calls. There should be an automated command to set this up.
- Max pods must be manually calculated and passed to kubelet for worker nodes. This should be automated, and will also allow custom networking to work with Managed Node Groups.
- Managed node groups now automatically calculates recommended max pods based on VPC CNI settings.
- ENIConfigs must be created for each availability zone. There should be an option to auto discover these subnets based on tags.

Many of these steps should be simplified and/or automated.

Additionally, documentation is limited. Should add more content like [this](https://aws.amazon.com/blogs/containers/eks-vpc-routable-ip-address-conservation/) to EKS docs.