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/requirements.txt

@@ -45,6 +45,12 @@ daphne
 
 gunicorn
 
+# -- THIRD PARTY APIS --
+# Deutsche Post International Shipping API client (local package)
+httpx>=0.23.0,<0.29.0  # Required by Deutsche Post client
+attrs>=22.2.0  # Required by Deutsche Post client
+python-dateutil>=2.8.0  # Required by Deutsche Post client
+
 # -- REST API --
 djangorestframework #REST Framework
 
@@ -90,6 +96,12 @@ faker #generates fake data for testing purposes
 
 zeep #SOAP tool
 
-## -- api --
+
+
+## -- API --
+
+#generates api (if schema exists (ONLY USE OPENAPI NO SWAGGER))
+openapi-python-client
+
 stripe
 gopay