feat(auth): Add support for Email MFA

[vincetran]

• [x] 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: https://github.com/aws-amplify/amplify-ui-android/pull/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
• [x] Yes (Please include a PR link for the documentation update) - PR Link Pending

General Checklist

• [x] Added Unit Tests
• [x] Added Integration Tests
• [x] 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.