[ADD] 収支のためのService及び実装クラスを作成しました。

src/main/java/com/example/demo/app/domain/service/TransactionService.java

@@ -0,0 +1,5 @@
+package com.example.demo.app.domain.service;
+
+public interface TransactionService {
+
+}

src/main/java/com/example/demo/app/domain/service/impl/TransactionServiceImpl.java

@@ -0,0 +1,10 @@
+package com.example.demo.app.domain.service.impl;
+
+import org.springframework.stereotype.Service;
+
+import com.example.demo.app.domain.service.TransactionService;
+
+@Service
+public class TransactionServiceImpl implements TransactionService{
+
+}