Add HTTP cache to API methods and add max timeout

Adds okhttp disk cache usage to polycentricandroid, as well as a 3 second timeout. A part of the current 10 second issue is select-non-in-cache items have to hit the server, and with the server still under pressure from non-updated backfill-bug-fixing-clients, sometimes they time-out. As more clients upgrade, hopefully even the 3 second timeout should be gone.

This only caches polycentric HTTP requests. Same steps can be modified to also cache other application uses.

This PR has to be called by a different PR in Grayjay itself, that also makes comment requests concurrent.