feat: add RequestContext to PubSubConn

[pabigot]

Add a wrapper that goes through the standard receiveInternal processing to match the API of the existing PubSubConn Receive methods.

Fixes: #592

[pabigot]

It doesn't seem that the design intent is to limit the features of PubSubConn, but an alternative approach that's only slightly less ideal would be to expose pubsub/receiveInternal as a visible, documented reply helper so we could do:

v := redis.PubSubReply(redis.ReceiveContext(c, ctx))

That might be useful anyway.

[stevenh]

Thanks for the PR