add post userbook

needed a refactor to check the id in db during dto validation

internal/mapper/userbookpostcreate.go

@@ -0,0 +1,13 @@
+package mapper
+
+import (
+	"git.artlef.fr/PersonalLibraryManager/internal/dto"
+	"git.artlef.fr/PersonalLibraryManager/internal/model"
+)
+
+func UserBookWsToDb(ub dto.UserBookPostCreate, user *model.User) model.UserBook {
+	return model.UserBook{
+		UserID: user.ID,
+		BookID: ub.BookID,
+	}
+}