Enable external ids

[aarongorka]

This also includes #55 , not sure what's going on there. Can we merge this if #54 isn't going to be merged?

[mbarneyjr]

Wanted to make a note that we'll review this and push an update to pypi. It might take a little while though, we're also working on an update to awsume, rearchitecting a lot of the core, but these changes will be incorporated to awsume v3 before the v4 release.

Thanks for the MR!

[mike-mosher]

+1 on this functionality. Would really help in our workflow

[mbarneyjr]

Just reviewed this, everything looks good except for in one spot

In awsumepy.py, lines 871-879 need to be swapped with lines 862-869. This is because a profile that has a session token in it wouldn't normally have an mfa_serial, role_arn, or source_profile, which triggers passes the condition that is checked before we check for the session token. I'll fix this after it's merged though, just an fyi

Thanks a ton for the MR! After I review the rest of the currently open MR's I'll release to pypi