feat(authenticator): Add ability to sign in with passwordless flow

[mattcreaser]

• PR conforms to Pull Request guidelines.

Issue #, if available:

Description of changes:
Allows user to complete a sign in via SMS OTP, Email OTP, password, or passkey via the USER_AUTH flow.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes TBD

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.

[mattcreaser]

This is an issue because you can still have Email MFA or SMS MFA enabled together with USER_AUTH, and if you do that then the codes that you get are 6 digits, not 8, so relying on the authenticationFlow is insufficient here.

The problem is that if you're using SMS OTP or Email OTP in a USER_AUTH sign in then the next step is CONFIRM_SIGN_IN_WITH_OTP and the code is 8 digits (long code), but if you're using Email MFA with USER_AUTH then the next step is CONFIRM_SIGN_IN_WITH_OTP and the code is 6 digits (short code).

I'm not sure there's a good way to be certain if it's a short code or long code.