Add SSL protection over gRPC between sqlflow client and server

[wangkuiyi]

As the client and server might cross the Internet.

On the contrary, the Jupyter magic command is at the server-side together wth the SQLFlow server, so no strict requirement of SSL.

[wangkuiyi]

To add SSL authentication to gRPC, we can do https://grpc.io/docs/guides/auth/

[wangkuiyi]

For more information about the deployment with service.pem, we need to refer to https://itnext.io/practical-guide-to-securing-grpc-connections-with-go-and-tls-part-1-f63058e9d6d1

The client needs the service.pem file if we self-sign the certificate of SQLFlow service.