Allow "assuming" temporary credentials

[ejholmes]

It would be nice if I could use assume-role to execute a binary with temporary credentials from GetSessionToken, like it can with AssumeRole. Unfortunately, this needs to happen upstream in the AWS SDK's first.

A use case would be to use assume-role to call GetSessionToken with the MFA token code first, then let another downstream binary assume roles with those creds, since it wouldn't need to know anything about MFA.