Add ProductInPult and BoughtInPult structs in api module

The new structures ProductInPult and BoughtInPult in api/model.go file have been added. These implementations are designed to facilitate the handling of product and purchasing data. Adding these types will help to establish a structure for the data flowing in and out of the API endpoints, and thereby improve the general utility and manageability of our system.

api/model.go

+package api
+
+import (
+	"git.sch.bme.hu/disappointment-industries/becskasszasch/db"
+	"time"
+)
+
+type ProductInPult struct {
+	Product *db.Product `json:"product"`
+	Amount  int64       `json:"amount"`
+}
+
+type BoughtInPult struct {
+	User           *db.User         `json:"user"`
+	ProductsInPult []*ProductInPult `json:"productsInPult"`
+	Date           time.Time        `json:"date"`
+	Pultosch       *db.User         `json:"pultosch"`
+}