request: Store client information using ZulipRequestNotes.

This concludes the HttpRequest migration to eliminate arbitrary
attributes (except private ones that are belong to django) attached
to the request object during runtime and migrated them to a
separate data structure dedicated for the purpose of adding
information (so called notes) to a HttpRequest.

zproject/backends.py

@@ -278,7 +278,9 @@ def rate_limit_auth(auth_func: AuthFuncT, *args: Any, **kwargs: Any) -> Optional
 
     request = args[1]
     username = kwargs["username"]
-    if not hasattr(request, "client") or not client_is_exempt_from_rate_limiting(request):
+    if get_request_notes(request).client is None or not client_is_exempt_from_rate_limiting(
+        request
+    ):
         # Django cycles through enabled authentication backends until one succeeds,
         # or all of them fail. If multiple backends are tried like this, we only want
         # to execute rate_limit_authentication_* once, on the first attempt: