[STSK13] Implement Sigv4 and sts:AssumeRole for AWSCredential class

[vexance]

Requesting the following changes to the AWSCredential class:

• getcred(): Should return an AWSCredential for the currently configured AUTH options. Currently it seems to return only the first credential created and does not update when the options update.
• session(): Similar to getcred(), the behavior instantiating a single boto3.Session object and then always returning that instance of the object should be update to always create a session for the current AUTH option creds
• sign_sigv4(): (New) Return an AWSSigv4 object from requests_auth_aws_sigv4 to use for signing requests.Request objects
• assume_role(): (New) Perform sts:AssumeRole and return an AWSCredential for those temproray creds

[vexance]

Completed with #15 . Closing