[EKS]: Support sts:SourceIdentity in configmap

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
The configmap supports role session name but not source identity. The role session name is an arbitrary string. The user can enter anything, masking their true identity. This can be addressed by using source identity which is immutable. It's set at first authentication, presumably from a reliable source such as a SAML assertion, and carries forward as the user assumes other roles. This attribute though is lost when using a role to gain access to EKS.

** Tell us what service**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Source identity provides a more reliable method to identity a user than role session name.

**Are you currently working around this issue?**
We're not! We have no choice but to use role session name which is easily thwarted.

**Additional context**
This should also be present in the kubernetes log in the extras section like role session name.

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

The issue identifies the EKS configmap and Kubernetes log extras as the affected entry points, but names no repository files or tests. Start by tracing how role session name is currently handled, then determine how immutable source identity should be retained through role access and exposed in the log extras; done means both behaviors are supported.

Written by the indexing model from the issue text.