Redis server-assisted client side caching

### Feature Request:[ Redis server-assisted client side caching](https://redis.io/topics/client-side-caching)
Envoy is an incredible tool that we've found critical for proxying redis connections, with the release of Redis 6, there seems to be an opportunity to take Envoy's redis capabilities to the next level by acting as an even faster local cache to Redis through the support of server-assisted client caching. Adding this capability to Envoy will make it simple to increase cache locality for many applications with a small, hot set of data, making it straightforward to build efficient sharded applications by handling asynchronous cache invalidation.

There would need to be substantial thought in terms of the memory profile, since it will shift Envoy from being a lighterweight proxy to also a relatively "thick" proxy-cache. I could imagine users tending to run a dedicated Envoy with a larger memory allocation separate from the lighterweight proxy variant for the purposes of isolation.

Regardless -- I really appreciate the work that Lyft, Matt Klein and all other contributors have put into this project, thank you all for the hard work 🙌