Authentication between client and server

[sverhoeven]

gRPC allows for SSL/TLS to be used for authentication between client and server. This means the client only accepts connections from a trusted server and a server only accepts messages from a trusted client.

This setup would be needed when model and notebook are communication over the Internet instead of a local network on the same machine.

See https://grpc.io/docs/guides/auth/

[sverhoeven]

See https://github.com/xenon-middleware/pyxenon for an implementation.