[feature] aws cred_process redirects stderr to /dev/tty such that users can see mfa prompts

[edulop91]

Summary

botocore will consume both stderr and stdout for their cred_process. We use this hack to make sure that stderr is displayed back to the user. stdout is still consumed by botocore as normal.

Test Plan

aws sts get-caller-identity --profile XXXXXX
INFO[0001] Requesting MFA. Please complete two-factor authentication with your second device             
INFO[0002] Device: XXXXXXX                               
{
    "Account": "XXXXXXXX", 
    "UserId": "XXXXXX", 
    "Arn": "arn:aws:sts::XXXX:assumed-role/XXXX/XXXX"
}

References

boto/botocore#1348

[codecov]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
=======================================
  Coverage   79.22%   79.22%           
=======================================
  Files          25       25           
  Lines        2026     2026           
=======================================
  Hits         1605     1605           
  Misses        278      278           
  Partials      143      143

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[ryanking]

lgtm, but @jgadling might have good feedback

[edulop91]

I'll apply @jgadling feedback in a followup pr