Make grpc client cache eviction period configurable

Envoy is currently hardcoded to evict thread-local GRPC clients from its cache if there is no activity for 50s. Due to the low QPS and high number of Envoy worker threads, it is very easy to cross this interval between consecutive requests on the same worker thread. Hence, many authorization requests from worker threads will incur a connection restablishment latency, resulting in high client-side authorization delays.

To avoid this issue, we would like to be able to use a longer cache eviction timeout. It would be preferable if this timeout is configurable per proxy.