[feature] 추상화 레이어 Calculate 추가

- 추상화 레이어를 활용하여 DI를 통해 유연한 테스트를 수행한다.

src/main/java/study/step_1/domain/Calculate.java

@@ -0,0 +1,5 @@
+package study.step_1.domain;
+
+public interface Calculate {
+    int splitAndSum(String express);
+}

src/main/java/study/step_1/service/AddCalculator.java

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+package study.step_1.service;
+
+import study.step_1.domain.Calculate;
+
+public class AddCalculator {
+
+    private final Calculate calculate;
+
+    public AddCalculator(Calculate calculate) {
+        this.calculate = calculate;
+    }
+
+    public int executeAdd(String express) {
+        return calculate.splitAndSum(express);
+    }
+}