Currently the NewServer function takes creates an internal gRPC server specifically for the use of pubsub. This is only useful if wanting to run go-libp2p-pubsub-grpc standalone, and not with other services.

Instead, we should modify the server functionality to act like a module instead, which lets this functionality easily be added into existing gRPC applications.

This pubsub module can then be registered against the provided gRPC server. This would also avoid needing to handle "closing" logic, as the parent process responsible for providing the gRPC server can handle shutting things down.