Expose Cluster Slots Commands to Downstream Clients

Redis cluster [protocol](https://redis.io/commands/cluster-slots/) suggests to return Cluster Slots command with `NULL` as endpoint address when running behind a load balancer.

This feature request is support CLUSTER SLOTS command to downstream clients with `NULL` as endpoint.

Our use case is to support existing redis cluster clients that are migrating to Envoy continue work in the same way using their existing redis clients like Lettuce.