DEV: API to register custom request rate limiting conditions (#30239)

This commit adds the `add_request_rate_limiter` plugin API which allows plugins to add custom rate limiters on top of the default rate limiters which requests by a user's id or the request's IP address.

Example to add a rate limiter that rate limits all requests from Googlebot under the same rate limit bucket:

```
add_request_rate_limiter(
  identifier: :country,
  key: ->(request) { "country/#{DiscourseIpInfo.get(request.ip)[:country]}" },
  activate_when: ->(request) { DiscourseIpInfo.get(request.ip)[:country].present? },
)
```

config/initializers/000-zeitwerk.rb

@@ -45,6 +45,7 @@ Rails.autoloaders.each do |autoloader|
     "http" => "HTTP",
     "gc_stat_instrumenter" => "GCStatInstrumenter",
     "chat_sdk" => "ChatSDK",
+    "ip" => "IP",
   )
 end
 Rails.autoloaders.main.ignore(