auth: support MFA without assuming a role

[JadenSimon]

Problem

It's possible to use MFA devices combined with long-term credentials but the Toolkit does not support this. Example configuration:

[default]
aws_access_key_id = access_key
aws_secret_access_key = secret_key
mfa_serial = arn:aws:iam::1234567890:mfa/user

No prompt is shown when selecting this profile in the Toolkit

Expected behavior

The Toolkit prompts for an MFA code and calls GetSessionToken on the user's behalf.