"As a System Administrator, I want to install a caching HTTP proxy between MediaWiki and the client, to lower the resource costs of frequent requests."

The Wikimedia Foundation servers have Varnish and/or Apache Traffic Server (ATS) as caching reverse proxy servers. One of the best parts of having a REST API is that we can support that caching infrastructure better.

To some extent, the caching proxy acts like an HTTP client, so much of the requirements in T238374 should help.

In addition, it will be important to:

• Modify the Cache-Control header to support the caching proxy (probably "public"?)
• Include active cache purging at create/update/delete time (there's a framework for this in MediaWiki already)
• Include a Vary header so that authorized requests (with Cookie or Authorization) are factored into the cache
• Include a Vary header so that other request headers that change the output, if any, are factored into the cache