Authenticator - Allow adding custom fields to SignIn screen

### 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.

Im setting up a multi-tenant SaaS that requires using and organizationId in order to login our users.

I've managed to implement a custom organization_id field on the SignUp form using the following code:

```

)
},
},
```

### 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 signIn formFields path and the handleSignIn service input shown in the issue; compare how sign-up custom userAttributes are passed with the sign-in flow. Done means a configured signIn organization_id reaches the service input, or the proposed component override supports the same flow.

Written by the indexing model from the issue text.