feat: Add MemberRepository (#3)

Author: youjin09222

server-toy-project/src/main/java/com/gdsc_teamb/servertoyproject/login/MemberRepository.java

@@ -0,0 +1,9 @@
+package com.gdsc_teamb.servertoyproject.login;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+import java.util.Optional;
+
+public interface MemberRepository extends JpaRepository<Member, Long> {
+    Optional<Member> findByMemberId(String username);
+}