Add configurable HTTP status code for Workhorse load shedding

What does this MR do and why?

Allow operators to configure a custom HTTP status code (e.g., 529) when Workhorse sheds load due to high Puma backlog. This enables distinguishing load shedding responses from other 503 errors caused by database timeouts or Gitaly issues, allowing NGINX to retry load shedding responses while not retrying other 503 errors.

Relates to gitlab-com/gl-infra/production#21451

How to set up and validate locally

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