feat: MemberService에 id로 존재 여부 확인 로직 추가

src/main/java/roomescape/member/business/MemberService.kt

@@ -36,5 +36,8 @@ class MemberService(
                             String.format("[email: %s, password: %s]", email, password),
                             HttpStatus.BAD_REQUEST
                     )
+
+    fun existsById(memberId: Long): Boolean = memberRepository.existsById(memberId)
+
 }