feat(user/jpa): creado método findByUsername en UserRepository

src/main/java/com/example/apprecetas/user/infrastructure/repository/mongodb/UserRepository.java

@@ -2,5 +2,8 @@ package com.example.apprecetas.user.infrastructure.repository.mongodb;
 
 import org.springframework.data.mongodb.repository.MongoRepository;
 
+import java.util.Optional;
+
 public interface UserRepository extends MongoRepository<UserDocument, String> {
+    Optional<UserDocument> findByUsername(String username);
 }