Add replica-only read mode to cluster and asyncio cluster

[johan-seesaw]

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Description of change

Introduce a new Enum and optional flag value to allow reading only from replicas, if the command supports it. It appears the sync version of RedisCluster used to support this when server_type was passed in to get_node_from_slot, but that parameter isn't set anywhere.

Additionally, it addresses the issue of reading from replicas while resharing, which can cause index failures.
It is an alternative, and more comprehensive solution to #2989, but in both sync and asyncio implementations. (#2988).