Add Apple signIn support

[JekaNS]

Added AppleSignInArgs.
After user authorized in WebView with AppleID, we can create OAuth credentials for Firebase using OAuthProvider("apple.com").credential(...)

Please pay attention that on windows platforms firebase_auth package was throw exception when trying use credentials with rawNonce.
It was fixed in firebase_auth: 5.1.4 PR#13086

Usage exmaple:

import 'package:desktop_webview_auth/desktop_webview_auth.dart';
import 'package:desktop_webview_auth/apple.dart';
import 'package:firebase_auth/firebase_auth.dart';  

final args = AppleSignInArgs(
    clientId: "your-service-id", //Services ID used as the web application identifier
    redirectUri: "https://your-project.firebaseapp.com/__/auth/handler",
);

try {
    final result = await DesktopWebviewAuth.signIn(args);
    if (result == null || result.idToken == null) {
      throw Exception("Authorize process terminated");
    }
} catch (err) {
    // something went wrong
}

final credential = OAuthProvider("apple.com").credential(
    idToken: result.idToken!,
    rawNonce: args.nonce,
);

FirebaseAuth.instance.signInWithCredential(credential);

[JekaNS]

Hi,
Any updates here? Can some one merge this PR?

Bug in firebase_auth about exception when pass credentials with rawNonce is fixed a year ago. firebase/flutterfire#13086