Istio external auth compability

[containerpope]

To make kelon work out of the box with Istio, we need to provide a GRPC endpoint for Istio external auth plugin. The details can be inspected here: https://github.com/open-policy-agent/opa-istio-plugin

This will allow us to forward auth requests from Istio Sidecars directly to the Kelon project without changing the service code to intercept incoming requests.

[containerpope]

@dcseifert Implementing this with Istio and a custom Envoy Filter rule is a dangerous process and should maybe be delayed in favor of other simpler implementations. We should try Gloo first. #42

[dcseifert]

Sounds like a good idea.