♻️(backend) extract throttling classes into a module

Extract throttling classes into a dedicated Python module, following the
structure of suitenumerique/docs.

This is a preparatory refactor to ease upcoming changes to the throttling
implementation. No functional behavior change is introduced in this commit.

src/backend/core/api/throttling.py

@@ -0,0 +1,14 @@
+"""Throttling modules for the API."""
+
+from rest_framework import throttling
+
+class RequestEntryAnonRateThrottle(throttling.AnonRateThrottle):
+    """Throttle Anonymous user requesting room entry"""
+
+    scope = "request_entry"
+
+
+class CreationCallbackAnonRateThrottle(throttling.AnonRateThrottle):
+    """Throttle Anonymous user requesting room generation callback"""
+
+    scope = "creation_callback"