[hw-1] implement cart service

cart/internal/domain/model/items.go

@@ -0,0 +1,16 @@
+package model
+
+import "fmt"
+
+type Item struct {
+	Product *Product
+	Count   uint32 `validate:"gt=0"`
+}
+
+func (i *Item) Validate() error {
+	if err := validate.Struct(i); err != nil {
+		return fmt.Errorf("invalid requested values: %w", err)
+	}
+
+	return nil
+}