feat(authenticator): Add support for Email MFA

[vincetran]

• PR conforms 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

The PR for the Amplify-Android change can be found here: aws-amplify/amplify-android#2935

This PR will be in draft mode until the above change has been released so that this PR can ingest the new version.

This PR adds the following new string resources:

<string name="amplify_ui_authenticator_button_continue">Continue</string>
<string name="amplify_ui_authenticator_mfa_setup_description">Select your preferred way to verify your identity when using two-factor authentication (2FA). This will be your default method for added account security.</string>
<string name="amplify_ui_authenticator_email_setup_description">Enter the email address you want to use for two-factor authentication (2FA). This email will receive verification codes when you sign in.</string>
<string name="amplify_ui_authenticator_mfa_selection_description">For added security, choose how you want to verify your identity for this sign-in. </string>
<string name="amplify_ui_authenticator_mfa_email">Email Message</string>
<string name="amplify_ui_authenticator_title_signin_continue_setup_mfa_select">Choose your preferred two-factor authentication method to set up</string>
<string name="amplify_ui_authenticator_title_signin_continue_email_setup">Add Email for Two-Factor Authentication</string>

How did you test these changes?
Manual testing, new and updated tests pass

Documentation update required?

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

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)
• Ensure commit message has the appropriate scope (e.g fix(liveness): message, fix(authenticator): message, fix(all): message)

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

[vincetran]

Your changes look good, @tylerjroach. I can't give an explicit approval because it's my PR.