Create distinct route for api/v4/internal/allowed with circuit breaker

What does this MR do and why?

This applies the circuitbreaker (implementation, MR) to the rails api/v4/internal/allowed endpoint.

Currently, requests to /api/* routes are proxied by this handler. This MR adds a handler specifically for the /api/v4/internal/allowed endpoint that wraps the proxy with a circuit breaker when the circuit breaker is enabled in the config.

References

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/525382

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Enable the circuit breaker feature flag Feature.enable(:workhorse_circuit_breaker)
• Add the following to your workhorse/config.toml

[circuit_breaker]
  enabled = true
  timeout = 30
  interval = 60
  max_requests = 1
  consecutive_failures = 5

• gdk restart
• git clone, push, pull via ssh with no issues
• Comment out this line
• Git clone, push, and pull trip the circuit breaker (gobreaker: state change in workhorse logs)
• Subsequent requests don't reach /allowed. You can add a breakpoint or a print statement here to ensure the endpoint isn't reached.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.