aws sts support

[nisabek]

The "problem" with temporary token renewal exists also in a "usual" aws sts setup (AWS Org, centralized users using "assumeRole" functionality with other accounts). Since I'm comfy with the afpre style already, I enhanced it with that flow. However I understand that this feature has nothing to do directly with Federation Proxy, so it's completely up to you if you want to merge, or let it live in fork...

Let me know if there are questions/comments/wishes.