implemented user service (no auth!)

backend-dart/lib/domain/repository/user_repository.dart

@@ -0,0 +1,17 @@
+import 'package:backend_dart/domain/entities/user.dart';
+import 'package:backend_dart/domain/interface/error.dart';
+import 'package:fpdart/fpdart.dart';
+
+abstract class UserRepository {
+  TaskEither<IError, User> create(User user);
+
+  TaskEither<IError, User> findByEmail(String email);
+
+  TaskEither<IError, User> findById(String id);
+
+  TaskEither<IError, User> update(User user);
+
+  TaskEither<IError, void> delete(String id);
+
+  TaskEither<IError, List<User>> findAll();
+}