Geo: Use a special header to tell Workhorse not to proxy current request

Proposal

Modify WorkHorse upstream.go logic to accept a specific Header that tells it not to proxy that specific request to a primary.

An example header we could use is: X-GitLab-Proxy-Skip: current / X-GitLab-Proxy-Skip: 1 / X-Gitlab-Proxy: false.

Additional Considerations

This will allow us to remove the hard-coded route exceptions later.