k3s module #4485

I've been doing a bit of hacking with running k3s in a container, in order to have a throwaway Kubernetes cluster for testing an operator.

Compared to previous experiences with trying to do the same with kind, k3s has been more pleasant. One key difference is the startup time, which is dramatically faster.

As there has been some historic demand for this kind of capability, I'll raise a PR to add the k3s module.