Add Hetzner DNS scripts

[Leo963]

Adds DNS challenge scripts for Hetzner DNS API.

For use the environment variable HETZNER_KEY must be set with an access token.

Also either HETZNER_ZONE_ID or HETZNER_ZONE_NAME must be set as well. When HETZNER_ZONE_ID is set it is directly used. When HETZNER_ZONE_NAME is set the zone id is obtained first.