Support redis custom commands in redis proxy

[VivekSubr]

Title: Support redis custom commands in redis proxy

Description:
We have redis filter in envoy: https://www.envoyproxy.io/docs/envoy/latest/configuration/listeners/network_filters/redis_proxy_filter

It supports all redis commands, maintaining a list of redis commands here: https://github.com/envoyproxy/envoy/blob/main/source/extensions/filters/network/common/redis/supported_commands.h

This is all good, however redis supports adding custom commands, and this filter is unable to proxy them... merely having a way to add the names of custom commands to the hash_set in supported_commands.h is sufficient to have this proxy support redis custom commands.

So this feature request boils down to - have a way to add strings to this hash_set in redis filter.

[KBaichoo]

cc @weisisea