[EKS] [request]: Allow Configuration of Pod CIDR

**Tell us about your request**
When using kubeadm to create a cluster you can pass --pod-cidr. This is required when networking with Flannel and some other networking plugins.

Most of the time we will just use Amazon CNI, but in some cases that is not an option due to pod density, pod timeliness etc. Flannel is a much better fit.

It is painful to either have to use gross hacks to continue with EKS, or use a whole other management strategy for the cluster just because we want to use Flannel. Philosophy of Kubernetes is flexibility AND Amazon CNI is not perfect fit for every use case so why force it as only way?

EKS should be able to accommodate using Flannel (or other networking) as advanced option.

**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?**
I want to be able to run Flannel, and cannot because control plane options are not exposed for me to define which keeps Flannel (and other network plugins) from being an option.

**Are you currently working around this issue?**
Run our own ETCD servers!!!

https://medium.com/@jeremy.i.cowan/running-flannel-on-eks-9a2f7a285a23

**Additional context**
Please just expose all the underlying control plane options, make it an advanced page in the web ui if you feel you have to hide it but locking down EKS so hard makes it quite inflexible and definitely less attractive as the one solution for deploying and managing kubernetes.

Start by reviewing the EKS control-plane options described in the issue and the kubeadm --pod-cidr requirement for Flannel, then read the linked Flannel-on-EKS article for context. Determine where EKS exposes networking configuration and define the supported scope for pod CIDR and alternative network plugins; done means the requested configuration path is specified and documented.

Written by the indexing model from the issue text.