feat: add firebase auth services

lib/services/auth_service.dart

@@ -0,0 +1,56 @@
+import 'dart:developer';
+
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:google_sign_in/google_sign_in.dart';
+
+enum SignInState {
+  authenticated,
+  disabled,
+  notFound,
+}
+
+class AuthService {
+  static final FirebaseAuth _auth = FirebaseAuth.instance;
+  static final GoogleSignIn _googleSignIn = GoogleSignIn();
+
+  /// Returns currently signed up user credentials.
+  static User? get getCurrentUser => _auth.currentUser;
+
+  /// Returns an instance of `FirebaseAuth`.
+  static FirebaseAuth? get getFirebaseAuthInstance => _auth;
+
+  static Future<User?> logOut() async {
+    try {
+      await _googleSignIn.signOut();
+      await _auth.signOut();
+    } on FirebaseAuthException catch (e) {
+      log(e.message!);
+    }
+    return getCurrentUser;
+  }
+
+  static Future<SignInState> signUp() async {
+    try {
+      GoogleSignInAccount? user = await _googleSignIn.signIn();
+
+      if (user != null) {
+        GoogleSignInAuthentication auth = await user.authentication;
+
+        AuthCredential credential = GoogleAuthProvider.credential(
+          idToken: auth.idToken,
+          accessToken: auth.accessToken,
+        );
+
+        await _auth.signInWithCredential(credential);
+      }
+    } on FirebaseAuthException catch (e) {
+      switch (e.code) {
+        case 'user-disabled':
+          return SignInState.disabled;
+        case 'user-not-found':
+          return SignInState.notFound;
+      }
+    }
+    return SignInState.authenticated;
+  }
+}