fix(authenticator): Add support for auto-sign-in

[mattcreaser]

• PR conforms to Pull Request guidelines.

Issue #, if available:

Description of changes:
This change fixes an issue where Authenticator would show an error after signup (requiring an app restart) if USER_AUTH has been enabled for the user pool, even if there is no intention to use that flow for sign in.

This is because having USER_AUTH enabled allows cognito to return the next sign up step of COMPLETE_AUTO_SIGN_IN which Authenticator could not handle. This PR adds support for this result.

How did you test these changes?

• Unit tests
• Test app

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.