Customise SignIn to have additional components

### 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.

Ability to Customise the SignIn component, similar to how we can customise the SignUp component with checkboxes etc. as shown here: [Example](https://ui.docs.amplify.aws/react/connected-components/authenticator/customization#sign-up-fields)

```
components={{
SignUp: {
FormFields() {
const { validationErrors } = useAuthenticator();

return (
<>
{/* Re-use default `Authenticator.SignUp.FormFields` */}

{/* Append & require Terms & Conditions field to sign up */}


);
},
},
}}
```

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

I was wondering if something similar can also be allowed for the SignIn. The use case is to ensure the users are validating each time they are signing in. Or something like a 'Remember Me' option when signing in which can be handles in a `validateCustomSignIn()` function.

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

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

Start with the React Authenticator customization documentation linked in the issue and compare the existing SignUp FormFields example with the SignIn component. Determine how additional SignIn fields and validation should work for the stated validation and “Remember Me” use cases; done means the requested SignIn customization is supported and documented.

Written by the indexing model from the issue text.