Enable rate limit per source IP without dropping requests

Follow-up 3 https://gitlab.com/gitlab-org/gitlab-pages/-/issues/490#note_676655191

Add configuration flags to Pages, integrate with the ratelimit package and add acceptance tests. The flags disable the rate limiter by default so this needs to be enabled in our environments manually. That means 1 MR for staging and another one for production.

Follow approach described in !575 (comment 675965682) where we deploy the code and start collecting metrics as-if-enabled to understand the impact of this functionality.

• pre-prod environments, blocked by https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/2083
• prod, blocked by gitlab-com/gl-infra/production#5706 (closed)
• create a dashboard in Grafana or Kibana