Geo: Cache `/api/v4/internal/geo_proxy` in Workhorse

To do

• Find out how gitlab-pages caches requests to the Rails API
  • Note that we can have concurrent requests come in at the moment that a cache expires. We don't want them all to do the internal API call.
• Cache the request to /api/v4/geo/proxy
• [-] Remove GEO_SECONDARY_PROXY environment variable. (Note that the endpoint is still behind the feature flag geo_secondary_proxy) => We should do this step after validation. I added it to #325732 (closed)
• Tests

Where does this fit in the epic?

Description	Issue	MR(s)
Add endpoint /api/v4/geo/proxy	#324664 (closed)	!63343 (merged)
Determine if this is a Geo Proxy by calling /api/v4/geo/proxy	#329670 (closed)	!60769 (merged)
If this is a Geo Proxy, then use the Geo Proxy routing table	#329672 (closed)	!63321 (merged)
Cache the result of the request	#329671 (closed) You are here	!66491 (merged)
[Feature flag] Rollout of geo_secondary_proxy and GEO_SECONDARY_PROXY	#325732 (closed)