[EKS] [request]: Ability to extract substring from ${OIDC_PROVIDER}:sub variable when using IRSA

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**Tell us about your request**
Ability to extract substring from `${OIDC_PROVIDER}:sub` variable when using [IRSA](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html)

**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 want to provide each pod with access to specific folder in S3 bucket where the folder name is automatically derived from the identity of the pod or service account name that is assigned to the pod. This way I can use a single role & policy without the need to create them per pod.
when using the `${OIDC_PROVIDER}:sub` variable in IAM policy associated with the assumed role, the returned value looks like this `system:serviceaccount::`. So all the folder names will have to have this structure. If I was able to obtain just the `namespace` or the `service account` I was able to name the S3 folders like I initially wanted.
If I could obtain a substring of the `sub` variable that would have been great. something like this:
```
"${OIDC_PROVIDER}:sub": "system:serviceaccount::"
"${OIDC_PROVIDER}:sub:namespace": ""
"${OIDC_PROVIDER}:sub:serviceaccount": ""
```

**Are you currently working around this issue?**
I need to create a dedicated role and policy per service account

Start by reviewing the issue’s EKS and IRSA context, including the `${OIDC_PROVIDER}:sub` value and the proposed namespace and service-account forms. This is a feature request for AWS IAM policy variable handling rather than a change tied to files or tests in the payload. Done would mean confirming support for extracting those substrings, or documenting the supported alternative.

Written by the indexing model from the issue text.