[feature] Support for RPC over Nats for proxy events

Nats provides a very efficient layer for RPC. We can add support for it in addition to HTTP/GRPC transports.

Also, I think it may be especially useful for https://github.com/centrifugal/centrifugo/pull/957 - as Nats may provide the most efficient layer for communication (400k RPC per second in my local experiments).