Add support for assuming a role in the provided AWS config

This is currently a limitation of the AWS C++ SDK and needs some changes to the [FDBAWSCredentialsProvider.cpp ](https://github.com/apple/foundationdb/blob/main/fdbclient/FDBAWSCredentialsProvider.cpp#L38), one of the issues in the AWS C++ SDK had a "solution": https://github.com/aws/aws-sdk-cpp/issues/150#issuecomment-538548438. It would be great to add support for assuming a role with the source_profile to allow such configurations to make use of the SDK auth.

There are some other limitation around the AWS C++ SDK that should be documented like: https://github.com/aws/aws-sdk-cpp/issues/1963.