Wrote OfferWriter and following classes to save offers to persistence

src/main/java/de/rwu/easydrop/api/dto/OfferDTO.java

@@ -0,0 +1,51 @@
+package de.rwu.easydrop.api.dto;
+
+import lombok.Data;
+import de.rwu.easydrop.model.Product;
+import java.util.Date;
+
+/*
+ * Offer data transfer object
+ */
+@Data
+public class OfferDTO {
+    /**
+     * Platform internal offer identifier.
+     */
+    private String offerId;
+
+    /*
+     * The product that our software buys
+     */
+    Product sourceProduct;
+
+    /*
+     * The product that our software sells
+     */
+    Product saleProduct;
+
+    /*
+     * Date of the creation of the offer
+     * NOTE: Use Timestamp? https://docs.oracle.com/javase/8/docs/api/java/sql/Timestamp.html
+     */
+    Date creationDate;
+
+    /*
+     * Date of last update of the offer on the destination website (API)
+     */
+    Date upDate;
+
+    /*
+     * Date of last check if offer is still valid
+     */
+    Date checkDate;
+
+    /**
+     * Creates OfferDTO instance.
+     *
+     * @param newOfferId  Internal Offer identifier
+     */
+    public OfferDTO(final String newOfferId) {
+        this.offerId = newOfferId;
+    }
+}