[NEW] Introduce a new client capability for throttling

[madolson]

With Valkey 8.0, we now have the ability for clients to indicate that they support a capability. One capability we discussed in Redis was the ability to shed load by sending an explicit throttle request. This is especially useful in caching cases, where the client would like to know they should go the backend database or API to fetch the data.

The main place we would like to send these requests is during a full-sync, where we may prefer to throttle incoming write requests instead of disconnecting replicas.

Related discussion on redis: redis/redis#12844.