terraform: Initial commit!

This patch adds the first iteration of the terraform definitions
required to deploy a Kubernetes cluster on Hetzner Cloud.

It installs the 3 node infrastructure and still needs quite some work,
but so far, it's a solid start. It's heavily inspired by `hcloud_okd4`,
that much, that is reuses the entire hcloud_instance module from there,
as well as various concepts.

References:
https://github.com/slauger/hcloud-okd4
https://git.shivering-isles.com/github-mirror/slauger/hcloud-okd4