Secure TLS-Connection for new MP-SPDZ Version

[kindlich]

In #15, the MP-SPDZ Version is changed to v0.2.8 which introduces TLS-secured communication between MP-SPDZ Clients (=ephemeral) and the MP-SPDZ Process.

The way the PR adds this is really rudimentary and insecure (e.g. the option InsecureSkipVerify is set to bypass certificate validation).

	tlsConfig := &tls.Config{
		Certificates:       []tls.Certificate{cert},
		InsecureSkipVerify: true,
	}

We will need to think about some points regarding the TLS connection:

• Who will create certificates (and re-created them when expired)?
  Types of connections
  • Ephemeral <-> MP-SPDZ Process (for transmitting secret shares/results)
  • Cross-Cloud MP-SPDZ instances (for computation)
• Will we continue using self-signed certificates or have a way to provide certifactes signed by CAs?
• Can we provide certificate using e.g. K8s Secrets instead of baking them into the base-image (do some parts of our infrastructure like Istio/Knative already provide some functionality for this here?)

[strieflin]

As discussed on Discord, the way to go is to make TLS channels optional in upstream MP-SPDZ and have authenticated channels implemented via Istio mechanisms.

[strieflin]

TLS has been switched off completely in #15 for both client/engine and engine/engine channels as it is not required in our setting.