This patch introduces a new provider for sops, this allows to use
sops-encrypted data to be used e.g. to input tokens and credentails as
well as other values into the setup. This should help to depend less on
koolbox environment variables.

References:
https://github.com/oauth2-proxy/oauth2-proxy/issues/2222

As I still maintain the demo instance, it's useful to integrate the demo
instance into the current terraform configuration, this allows it to
benefit from further developement of this repository.

The current README and variable descriptions should help anyone
interested in creating their own demo instance equivalent, to deploy
this to their hetzer cloud account. Even if not, it might provide some
nice inspiration regarding the instance handling.

This patch drops the unused subnet variable from the hcloud_instance
module, which wasn't used anyway. This help to make the module fully
compatible with the way the hedgedoc demo instance is deployed.