PR Feature/ new /verify-request for nginx

[alexandru-m-g]

This is based on @teodorescuserban's idea. The problem that we're trying to solve is related to the nginx cache. When nginx was serving a request from the cache, the app_identifier was no longer checked because the request was not reaching HAPI.

Before responding to a request from cache, nginx will now first check with HAPI that the app_identifier provided in the request is correct.
This happens by nginx sending a request to /api/util/verify-request . The endpoint doesn't really do anything, but if it gets to return HTTP code 200 with an empty body then nginx knows it's allowed to serve the request from the cache.

The request will have the same HTTP headers as the original request received by nginx + it will contain an additional header X-Original-URI that will contain the URL of the original request.

With the added complexity, I had to refactor the app identifier middleware to be a bit more readable

[IanHopkinson]

Thanks for the explanation in the PR opening - that was really useful for this one.

Technically I don't have much to contribute on this, my only question was whether the new verify-request endpoint would appear in the docs UI - my reading is that it doesn't because of the include_in_schema=False entries