[EKS] [request]: Support thin tokens for external identity providers

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**Tell us about your request**
Allow the EKS Identity Provider to authenticate external OIDC using "thin" tokens.

**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?**
The implementation available today on EKS assumes that tokens returned are "fat" (i.e. containing both the token itself and supporting information like group claims). It is common for IDP's to present only a "thin" token (i.e. only containing the token) and expect a second query to something like `userinfo/` to gather further information about the user. It would be helpful as an advanced feature to be able to configure this resource in order to use thin tokens.

**Are you currently working around this issue?**
We use a two step process.The first to login to AWS and obtain session tokens, and then the second to acquire a token into Kubernetes itself via `aws eks get-token`.

The issue provides no repository files or tests; begin by tracing the EKS Identity Provider's OIDC flow and the aws eks get-token workaround described. Done means the EKS resource can be configured for thin tokens and obtain supporting user information through userinfo/.

Written by the indexing model from the issue text.