fix: add MultiplyFunction (#5)

Adds MultiplyFunction implementing CalcFunction, following the
Add/Subtract pattern, wired into LoopTestApplication.

src/main/java/com/example/LoopTestApplication.java

@@ -2,6 +2,7 @@ package com.example;
 
 import com.example.util.AddFunction;
 import com.example.util.CalcFunction;
+import com.example.util.MultiplyFunction;
 import com.example.util.SubtractFunction;
 
 public class LoopTestApplication {
@@ -12,5 +13,7 @@ public class LoopTestApplication {
         System.out.println("ADDITION: " + add.calc(2, 5, 6));
         CalcFunction subtract = new SubtractFunction();
         System.out.println("SUBTRACTION: " + subtract.calc(2, 5, 6));
+        CalcFunction multiply = new MultiplyFunction();
+        System.out.println("MULTIPLICATION: " + multiply.calc(2, 5, 6));
     }
 }