redis: Send TRYAGAIN if key not found in MGET request

*Title*: Send TRYAGAIN if key not found in MGET request

*Description*:

When an MGET request is issued, `redis_proxy` send `GET` request on each key and collects the response [here](https://github.com/envoyproxy/envoy/blob/main/source/extensions/filters/network/redis_proxy/command_splitter_impl.cc#L278-L310).

As per redis cluster spec [here](https://redis.io/commands/cluster-setslot/#:~:text=If%20the%20command,can%20be%20executed.) , if a hash slot is in migrating state and if any of the keys are not found, the response to the downstream client should be `TRYAGAIN`.

We run `redis_proxy` on server side, however it seems this part of the code is common if `redis_proxy` is executed on client or server side. In this case the redis server needs to respond with `TRYAGAIN` for the `MGET` request but today individual `GET` request prohibits this behavior.