Customize cillium cni configuration

**What would you like to be added**:
As mentioned in https://github.com/aws/eks-anywhere/issues/362, we want the ability to be able to modify configuration of the cni being installed (for example cilium in this case) before installing it on EKS-A. Currently, EKS-A deploys fixed version of cilium that can only be modified after the fact, which is currently not supported either.

**Why is this needed**:
Different users may have different use cases for configuration when deploying a cni on their EKS-A cluster. The option for debugging is a possibility as well for why a user might want to edit the cni configuration beforehand.

Start by reading issue #362 and tracing the existing EKS-A CNI installation flow. Define how users would provide Cilium configuration before installation and how that configuration would be applied; done means a documented, supported customization path with appropriate validation and coverage.

Written by the indexing model from the issue text.