Support for AWS_STS_REGIONAL_ENDPOINTS

[BharatKJain]

When the env AWS_STS_REGIONAL_ENDPOINTS is set to regional, it's calling sts.amazonaws.com whereas it should call sts.ap-south-1.amazonaws.com or some other regional endpoint.

AWS docs reference

This helps when we're haskell code running in a closed environment where there's no internet available to make the STS calls, and regional endpoints can be used using VPC endpoints service which allows direct connectivity to some services.