support session tokens

[wwalker]

In order to use IAM instance profiles, one must pass the session token.

It should be as simple as passing it as a header:

x-amz-security-token:FwoGZXIvYXdzEMn...long-string-of-base64-encoded-gibberish...Tsaf=

[anton7c3]

Can be solved with https://github.com/taylorfinnell/awscr-s3/pull/114 and the usage of an external library to obtain the credentials: https://github.com/y2k2mt/aws-credentials.cr