OPA-based function auth

[pmundt]

We have prepared an OPA-based function authentication mechanism for OpenFaaS, which would be great to have referenced in the list of available auth mechanisms for those that may be interested in this. The implementation can be found here:

https://github.com/adaptant-labs/openfaas-function-auth-opa

From a quick start point of view, it's useful to have an OPA server deployed as part of the faas stack, but this is not strictly necessary. We've illustrated this for convenience here:

adaptant-labs/faas@a0d201c

We plan to further modify the mechanism such that each function can include its own policy definition, and will prepare a template to this effect in the coming days - after which we'll probably also write a blog about this to help others get started with OpenFaaS/OPA integration.

[pmundt]

@alexellis any feedback?