Add option to rate limit SSH requests

Problem to solve

Customers have found that large volumes of SSH requests to their GitLab instance can trigger excessive load.

Adding an option to allow admins to rate limit SSH requests per IP would allow them to more easily keep their instance stable and performant.

Target audience

Product Manager

Further details

Currently we have the ability to set a rate limit by IP and User for non-SSH requests in the Admin Area.

Proposal

Add new options under Admin Area -> Settings -> Network -> User and IP Rate Limits for SSH.

What does success look like, and how can we measure that?

Links / references

ZenDesk ticket 111430 (Internal Link)

/cc @jeremy