WEBMESS-1591: Add StepUp conversation support with signIn command and events.

[ranjithsai]

1. Add the AuthProvider.signIn event. Brand will implement this command in their end and publish signedIn event.
2. When brand signIn fails, they must publish signInFailed event (explained in developer docs).

[ranjithsai]

@ININDevEvangelists checking: Is there a way I can add my PM's here as reviewers?

[timsmithgenesys]

@ranjithsai if the PM completes required security compliance training, they can request access to the repo. Once they have access, you can add them as a reviewer.