[Feature Request] Amplify.Auth.signUpWithWebUI

Right now `Amplify.Auth.signInWithWebUI` only opens to the login page. The customer can always click "Sign up", but that text is hidden at the bottom of the page and not very noticeable.

I'd like to request a `Amplify.Auth.signUpWithWebUI` endpoint, that simply opens to the Sign Up page by default. That way I can have Login / Register buttons of my own that trigger `signInWithWebUI` and `signUpWithWebUI`.

I've dug through the source code and couldn't figure out a way to do this already. If it is already possible sorry for the ticket but please let me know how.

Start at the existing Amplify.Auth.signInWithWebUI entry point and trace how it chooses the initial page. Define the corresponding signUpWithWebUI API and confirm that it opens the Sign Up page by default, while preserving the existing sign-in behavior. Done means callers can use separate Login and Register buttons through the two endpoints.

Written by the indexing model from the issue text.