[EKS] [request]: Support Kubernetes authentication proxy mode

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**Tell us about your request**
Add support for [authenticating proxies](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#authenticating-proxy) on EKS clusters.

**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?**
We currently use authenticating proxies for a few reasons:
1. Allow us to authenticate users/services based on their X.509 certificates and attach dynamic group information from our LDAP system (we don't need to update certs as LDAP membership changes/expires)
2. Allow us to monitor and control all traffic being send to our kube-apiservers. This allows us to grant custom rate limits to different users and if we have a bad client/integration we are able to rate limit/cut off their traffic without impacting the rest of our users (we still use Kubernetes rate limits as a last resort rate limit).
3. Allow us to have custom deployment orchestrators that deploy on-behalf of other users while still using the end-user identity/permissions to create and manage the changes.

The current authentication models EKS supports make it hard (or infeasible) to support of use cases above. Supporting authentication proxies would allows to more easily start using EKS / migrate from self-hosted clusters without deprecating our current use cases.

Start with the Kubernetes authenticating proxy documentation linked in the issue, then compare its requirements with the authentication models currently supported by EKS. This is a roadmap-level request rather than a scoped repository change; done means EKS supports authenticating proxies for the certificate, dynamic-group, traffic-control, and delegated-orchestration use cases described.

Written by the indexing model from the issue text.