Support for case insensitive properties in the aws credentials file

Originally reported in https://github.com/aws/aws-sdk-java-v2/pull/4164.

Some SDKs (like Java, .NET, Javascript, Ruby) only support lowercase attributes in the aws `credentials` file, like:

```
[default]
aws_access_key_id=
aws_secret_access_key=
aws_session_token=
```

Uppercase properties like `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` are ignored by most SDKs - although it works fine in AWS CLI.

This is a feature request for support of case insensitive `credentials` file properties , in a consistent manner across all the SDKs.

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