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? },
)
```
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