refactor code, add interfaces for services

src/main/java/com/faf223/expensetrackerfaf/service/ITransactionService.java

@@ -0,0 +1,14 @@
+package com.faf223.expensetrackerfaf.service;
+
+import com.faf223.expensetrackerfaf.model.IMoneyTransaction;
+
+import java.util.List;
+
+public interface ITransactionService {
+
+    void createOrUpdate(IMoneyTransaction transaction);
+    List<? extends IMoneyTransaction> getTransactions();
+    List<? extends IMoneyTransaction> getTransactionsByEmail(String email);
+    IMoneyTransaction getTransactionById(long id);
+
+}