add: order model, order create api, order create serializer, order item model

core/apps/orders/admin/__init__.py

@@ -0,0 +1 @@
+from .order import *

core/apps/orders/admin/order.py

@@ -0,0 +1,14 @@
+from django.contrib import admin 
+
+from core.apps.orders.models.order import Order, OrderItem
+
+
+class OrderItemInline(admin.TabularInline):
+    model = OrderItem
+    extra = 0
+
+
+@admin.register(Order)
+class OrderAdmin(admin.ModelAdmin):
+    list_display = ['id', 'user', 'total_price']
+    inlines = [OrderItemInline]