Support for STS Interface Endpoints

**What would you like to be added**:
Support for STS interface endpoints in the EKS pod identity webhook, which would allow an STS endpoint to be injected into the pod's environment variable that can be used by AWS SDKs and the AWS CLI to make the `sts:AssumeRoleWithWebIdentity` call. Note this would also require changes to the SDKs and CLI. Requiring all users of this webhook to set `private_dns` to true is unreasonable.

**Why is this needed**:
Security conscious users don't want their pods talking to the STS endpoint over the internet, since this forces their pods to have internet access. Even if their services are user facing, their pods don't need to be. Having this security posture lowers the risk of sensitive data being exfiltrated.

Start by reviewing the EKS pod identity webhook's current endpoint injection behavior and the AWS SDK and CLI requirements for STS interface endpoints. The issue names no files or tests, and notes that SDK and CLI changes are also required. Done means pods can use an injected STS endpoint for AssumeRoleWithWebIdentity without requiring public internet access.

Written by the indexing model from the issue text.