Change Redis cluster discovery to use CLUSTER_SHARDS API

Title: Change Redis cluster discovery to use CLUSTER_SHARDS API

Description: Currently Envoy uses CLUSTER_SLOTS to fetch cluster topology from Redis. CLUSTER_SLOTS is deprecated in Redis 7.0. It is slower and less efficient than the new API, CLUSTER_SHARDS. We observed CLUSTER_SHARDS payload size to be 10x smaller than the CLUSTER_SLOTS payload for big redis cluster with 12k shards.

Redis doc also recommends using CLUSTER_SHARDS over CLUSTER_SLOTS

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