[FL-34] Use custom CNI for cluster create operation

Currently, EKS-A supports replacing the default CNI with custom option through cluster upgrade: https://anywhere.eks.amazonaws.com/docs/clustermgmt/networking/cluster-replace-cilium/

It would be nice to be able to specify the custom CNI during cluster creation so that user does not need to manually swap it after cluster is created.

Start with the linked cluster-replace-cilium documentation to understand the existing custom-CNI workflow during upgrade. Then trace the cluster creation path and define how custom CNI selection should be supplied at creation time; done means a cluster can be created with the custom CNI without a manual post-creation swap.

Written by the indexing model from the issue text.