Allow configuring redis instance for rate limiting

What does this MR do?

The ability to use a separate Redis for rate-limiting (RackAttack and Application Rate Limits) is being added (gitlab-com/gl-infra&526 (closed)), and this MR supports that by enabling configuration of such a Redis in Helm.

The Rails MR is still in flight but the only dependency is the name of the config yaml, which is "rate_limiting.yml" and I would not expect that to undergo any discussion/disagreement.

Related issues

• gitlab-com/gl-infra&526 (closed)
• gitlab-org/gitlab!70414 (merged)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
  • This redis instance isn't used yet, but will be inhttps://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1247 which will contain tests
• Equivalent MR/issue for omnibus-gitlab opened
  • gitlab-org/omnibus-gitlab!5624 (merged)