Add configurable rate limiting for the creation of rooms. (#18514)

Default values will be 1 room per minute, with a burst count of 10.

It's hard to imagine most users will be affected by this default rate,
but it's intentionally non-invasive in case of bots or other users that
need to create rooms at a large rate.
Server admins might want to down-tune this on their deployments.

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

schema/synapse-config.schema.yaml

@@ -2228,6 +2228,17 @@ properties:
     examples:
       - per_second: 2.0
         burst_count: 20.0
+  rc_room_creation:
+    $ref: "#/$defs/rc"
+    description: >-
+      Sets rate limits for how often users are able to create rooms.
+    default:
+      per_user:
+        per_second: 0.016
+        burst_count: 10.0
+    examples:
+      - per_second: 1.0
+        burst_count: 5.0
   federation_rr_transactions_per_room_per_second:
     type: integer
     description: >-