feat : Create UserRepository class

src/main/java/umc/spring/post/repository/UserRepository.java

+package umc.spring.post.repository;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+import umc.spring.post.data.entity.User;
+
+import java.util.Optional;
+
+public interface UserRepository extends JpaRepository<User, Long> {
+    Optional<User> findByUserId(String userId);
+}