Feature: add role assumption support

[zixu-zhao]

Hi,

This feature is about to support the role assumption credentials of aws-sdk. You could find more details from this link. Our company now requires this feature, so we fork the project by ourselves and modify the source code to make it work. However, we would appreciate and glad to see if you could officially support this feature, so I make this pull request.

Please let me know if you need more change or improvement in this pull request, and I would be glad to complete this feature if applicable. Thanks!