Feat: Order 도메인 객체 구현

- totalPrice를 계산하는 calculateTotalPrice() 구현
- deliveryPrice를 계산하는 calculateDeliveryPrice() 구현

BE/src/main/java/sidedish/com/domain/Order.java

@@ -0,0 +1,29 @@
+package sidedish.com.domain;
+
+import lombok.Getter;
+
+@Getter
+public class Order {
+	private final Product product;
+	private final long count;
+	private final long totalPrice;
+	private final long deliveryPrice;
+
+	public Order(Product product, long count) {
+		this.product = product;
+		this.count = count;
+		this.totalPrice = calculateTotalPrice();
+		this.deliveryPrice = calculateDeliveryPrice();
+	}
+
+	private long calculateTotalPrice() {
+		return count * product.getFixedPrice();
+	}
+
+	private long calculateDeliveryPrice() {
+		if (totalPrice >= product.getDeliveryPolicy().getFreeDeliveryOverAmount()) {
+			return 0;
+		}
+		return product.getDeliveryPolicy().getDeliveryCharge();
+	}
+}