create promocode page

lib/domain/usecase/apply_promo_code_usecase.dart

@@ -0,0 +1,12 @@
+import '../entities/promo_code_result.dart';
+import '../repositories/promo_code_repository.dart';
+
+class ApplyPromoCodeUsecase {
+  final PromoCodeRepository repository;
+
+  ApplyPromoCodeUsecase(this.repository);
+
+  Future<PromoCodeResult> call(String code) {
+    return repository.applyPromoCode(code);
+  }
+}