Support keyspace notifications (Khg and more)

[Guillermogsjc]

it would be very nice to have all kind of key notifications supported, for start this use case is pretty useful

notify-keyspace-events Khg

for plenty of cases (RT application websocket endpoint with pubsub to send only on key modifications for example).

This is related to expire event already supported

#1657

but imho ideally all the event notifications API would be nice to be developed, as it is a great tool inside redis

https://redis.io/docs/latest/develop/use/keyspace-notifications/

and in fact, in multithreaded version like dragonfly, it would penalise much less to have plenty of suscriptions :)