Add CartDAOMySQLImpl class, implement CartDAO interface

Signed-off-by: Minh Vương <hardingadonis@gmail.com>

src/main/java/io/hardingadonis/r7/services/Singleton.java

@@ -9,6 +9,8 @@ public class Singleton {
     public static DBContext dbContext;
 
     public static AdminDAO adminDAO;
+
+    public static CartDAO cartDAO;
 
     public static CategoryDAO categoryDAO;
 
@@ -20,6 +22,8 @@ public class Singleton {
         dbContext = new DBContextMySQLImpl();
 
         adminDAO = new AdminDAOMySQLImpl();
+
+        cartDAO = new CartDAOMySQLImpl();
 
         categoryDAO = new CategoryDAOMySQLImpl();