feat: add email option for mfa

[elishagreenwald]

This PR adds an option to use email as a factor for MFA

Description

Currently, email isn't supported as a factor for MFA. This PR adds email as a factor. Its pretty much the same as sms coding wise and I just had to make a few changes. I added tests as well.

Related Issue

https://github.com/Nike-Inc/gimme-aws-creds/issues/394

Motivation and Context

I use email for MFA.

How Has This Been Tested?

I tested locally

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.