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>

docs/usage/configuration/config_documentation.md

@@ -1996,6 +1996,31 @@ rc_reports:
   burst_count: 20.0
 ```
 ---
+### `rc_room_creation`
+
+*(object)* Sets rate limits for how often users are able to create rooms.
+
+This setting has the following sub-options:
+
+* `per_second` (number): Maximum number of requests a client can send per second.
+
+* `burst_count` (number): Maximum number of requests a client can send before being throttled.
+
+Default configuration:
+```yaml
+rc_room_creation:
+  per_user:
+    per_second: 0.016
+    burst_count: 10.0
+```
+
+Example configuration:
+```yaml
+rc_room_creation:
+  per_second: 1.0
+  burst_count: 5.0
+```
+---
 ### `federation_rr_transactions_per_room_per_second`
 
 *(integer)* Sets outgoing federation transaction frequency for sending read-receipts, per-room.