[EKS] [request]: Support getting EKS token without AWS CLI

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**Tell us about your request**

Currently the only ways to get a token to authenticate with the Kube API server is to either run `aws eks update-kubeconfig` or `aws eks get-token`. I am requesting that the latter be turned into a standard AWS EKS API operation that can be called through any AWS SDK.

**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 don't like having the AWS CLI in the critical path of EKS automation I'm writing. We've been burned too often with writing a project that we think is self-contained with no dependencies but actually implicitly depends on the AWS CLI. Additionally, the AWS CLI isn't that fast (which is fine for a CLI, but is annoying for automation that we write).

**Are you currently working around this issue?**
Just using `aws eks get-token`

**Additional context**
I'm somewhat surprised that `get-token` isn't already an API method, it feels like it should be.

Start with the existing aws eks get-token and aws eks update-kubeconfig flows, then review how EKS authentication reaches the Kubernetes API server. Done means defining and delivering an EKS API operation that provides the token through an AWS SDK without requiring the AWS CLI.

Written by the indexing model from the issue text.