FR (Authenticator): Sign with Github/Discord

### 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.

I try to find a way to add a button like "Sign in with Google" but with Github.

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The social providers are [hard coded](https://github.com/aws-amplify/amplify-ui/blob/main/packages/react/src/components/Authenticator/FederatedSignIn/FederatedSignIn.tsx#L30) and I do not find a way to extend it.

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

_No response_

### 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 by reading packages/react/src/components/Authenticator/FederatedSignIn/FederatedSignIn.tsx, especially the hard-coded social provider list. Trace how the Authenticator socialProviders property reaches that entry point, then confirm that GitHub and Discord can be configured and rendered as sign-in options.

Written by the indexing model from the issue text.