Profile endpoint rate limit (#20218)

This provides configurable (via `rc_profile`) rate limits for profile
endpoints:
- `GET /profile/{username}`
- `GET /profile/{username}/{keyName}` including (`displayname` &
`avatar_url`)

### Pull Request Checklist

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---------

Co-authored-by: Erik Johnston <erik@matrix.org>

docs/usage/configuration/config_documentation.md

@@ -2116,6 +2116,27 @@ rc_user_directory:
   burst_count: 200.0
 ```
 ---
+### `rc_profile`
+
+*(object)* This option allows admins to ratelimit profile lookups by clients.
+
+Requests are limited per user when the request is authenticated, otherwise per client IP address.
+
+_Added in Synapse 1.162.0._
+
+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_profile:
+  per_second: 1.0
+  burst_count: 500.0
+```
+---
 ### `federation_rr_transactions_per_room_per_second`
 
 *(integer)* Sets outgoing federation transaction frequency for sending read-receipts, per-room.