WLCG standard system JWKS cache

Goal: Provide a system-wide cache of JWKS information for issuers of interest that multiple token-parsing libraries can reuse. By having a cache in a trusted location on the system (and allowing the library to trust the system), we can reduce the impact of temporary outages of the JWKS endpoint, support clients that do not have direct access to the internet, reduce the load on the endpoint, and avoid rate-limiting conditions. This document outlines the motivation behind the design choices and the design of the cache.