add rate service

lib/domain/entities/discount_by_weight.dart

@@ -0,0 +1,23 @@
+class DiscountByWeight {
+  String id;
+  double weight;
+  double discount;
+
+  DiscountByWeight({this.id, this.weight, this.discount});
+
+  factory DiscountByWeight.fromMap(Map<String, dynamic> map, String id) {
+    return DiscountByWeight(
+      id: id,
+      weight: (map['weight'] ?? 0).toDouble(),
+      discount: (map['discount'] ?? 0).toDouble(),
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      "id": id,
+      'weight': weight,
+      'discount': discount,
+    };
+  }
+}