feat(auth): Add Passwordless features to Amplify

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:
This PR adds support for Cognito's new Passwordless Sign In Flows.

This includes the following new functionality for the Auth category:

• Passwordless Sign-Up using a One Time Passcode (OTP) sent to a user's email address or SMS number
• Auto-Sign In feature to sign in directly after signing up, without needing to re-enter the user's information
• New USER_AUTH flow that can be used to sign in with any of the following mechanisms
  • Password & Password SRP
  • Email OTP
  • SMS OTP
  • WebAuthn
• WebAuthn credential management APIs
  • Register device as a WebAuthn credential, allowing user to sign in with biometrics
  • List the registered WebAuthn credentials for the current user
  • Remove a registered WebAuthn credential

Please see the associated docs PR for a more comprehensive overview of these new features.

How did you test these changes?

• Integration tests
• Unit tests
• Manual verification

Documentation update required?

• No
• Yes: add passwordless docs#8122

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.