Merge zhuge/feature/coupon-api into main branch

Finish basic coupon api and merge into main branch.

core/api/coupon.py

+from rest_framework import viewsets
+from rest_framework.permissions import IsAuthenticated
+
+from core.serializers.coupon import CouponSerializer
+from core.models.coupon import Coupon
+
+
+class CouponViewSet(viewsets.ReadOnlyModelViewSet):
+    permission_classes = (IsAuthenticated,)
+    serializer_class = CouponSerializer
+    queryset = Coupon.objects.all()