feat(auth): Add support for Email MFA

[vincetran]

• PR title and description conform to Pull Request guidelines.

Issue #, if available: N/A

Description of changes:
The PR aims to add support for Email MFA that has been released by Cognito.
https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-settings-advanced-security-email-mfa.html

Draft PR for the Authenticator UI change: aws-amplify/amplify-ui-android#199

How did you test these changes?
Manual testing, new and existing unit tests pass, new and existing integration tests pass

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update) - PR Link Pending

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.