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? },
)
```
There have been too many flaky tests as a result of leaking state in
Redis so it is easier to resolve them by ensuring we flush Redis'
database.
Locally on my machine, calling `Discourse.redis.flushdb` takes around
0.1ms which means this change will have very little impact on test
runtimes.
