New Example - Firebase Google Authentication (#106)

* Added the firebase+google_authentication example

* Modified Readme.md

firebase_google_authentication/lib/Services/google_auth.dart

@@ -0,0 +1,49 @@
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:google_sign_in/google_sign_in.dart';
+
+
+class GoogleSignInProvider extends ChangeNotifier{
+
+  final googleSignIn = GoogleSignIn();
+
+  GoogleSignInAccount? _user;
+  User? user1;
+  GoogleSignInAccount get user => _user!;
+  String? userId;
+  FirebaseAuth auth = FirebaseAuth.instance;
+
+  Future googleLogin()async{
+    try{
+      final googleUser = await googleSignIn.signIn();
+      if(googleUser == null) return;
+      _user = googleUser;
+
+      final googleAuth = await googleUser.authentication;
+
+      final credentials = GoogleAuthProvider.credential(
+        accessToken: googleAuth.accessToken,
+        idToken: googleAuth.idToken,
+      );
+
+      final UserCredential authresult= await FirebaseAuth.instance.signInWithCredential(credentials);
+      notifyListeners();
+    }catch(e){
+      print(e.toString());
+    }
+
+  }
+
+  Future googleLogout()async{
+    try{
+      print('Logging Out');
+      await googleSignIn.disconnect();
+      FirebaseAuth.instance.signOut();
+    }catch(e){
+      FirebaseAuth.instance.signOut();
+    }
+    notifyListeners();
+  }
+
+
+}