FR (Authenticator): Customize whether federated providers or username/password are shown on SignIn/SignUp

### On which framework/platform would you like to see this feature implemented?

React

### Which UI component is this feature-request for?

Authenticator

### Please describe your feature-request in detail.

Our app uses cognito for user management but it only allows signing in/up with a social provider. We would like to disable the username/password input on the sign in page, only showing social providers. I looked into doing this with css but I don't think there is a css selector for the login group.

### Please describe a solution you'd like.

An option for authenticator called `hideSignIn` or something similar which hides the username/password flow, only showing the federated providers.

### We love contributors! Is this something you'd be interested in working on?

- [X] 👋 I may be able to implement this feature request.
- [ ] ⚠️ This feature might incur a breaking change.

Start at the React Authenticator component and review how its sign-in and sign-up views choose between federated providers and username/password. Confirm the desired option naming and behavior for both views; done means an explicit configuration can hide the username/password flow while preserving federated sign-in.

Written by the indexing model from the issue text.