Integrate Deutsche Post shipping API and models

Added Deutsche Post as a shipping carrier, including new models, admin, serializers, and API client integration. Updated Carrier and SiteConfiguration models to support Deutsche Post, including shipping price and API credentials. Added requirements for the Deutsche Post API client and dependencies.

backend/thirdparty/deutschepost/apps.py

@@ -0,0 +1,6 @@
+from django.apps import AppConfig
+
+
+class DeutschepostConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'thirdparty.deutschepost'