request: Refactor ZulipRequestNotes to RequestNotes.

This utilizes the generic `BaseNotes` we added for multipurpose
patching. With this migration as an example, we can further support
more types of notes to replace the monkey-patching approach we have used
throughout the codebase for type safety.

zerver/lib/rate_limiter.py

@@ -42,10 +42,10 @@ class RateLimitedObject(ABC):
         )
 
     def rate_limit_request(self, request: HttpRequest) -> None:
-        from zerver.lib.request import get_request_notes
+        from zerver.lib.request import RequestNotes
 
         ratelimited, time = self.rate_limit()
-        request_notes = get_request_notes(request)
+        request_notes = RequestNotes.get_notes(request)
 
         request_notes.ratelimits_applied.append(
             RateLimitResult(