Geo: Secondary sites proxy to primary site

• Make Workhorse call /internal/geo_proxy.
• When it returns a geo_proxy_url, then switch to Geo-secondary-specific routing very similar to gitlab-workhorse!688 (diffs).
• Do we need to cache this in Workhorse? => Yes

Note: Workhorse development happens in the main GitLab codebase now! 🎉