[wip] platform: add Hetzner

[tormath1]

This PR brings a boilerplate to add Hetzner support. API implementation is missing (i.e interaction between Mantle and Hetzner). For now, the platform/api/hetzner/api.go is missing some functions implementation.

The current usage would be the following:

HETZNER_IMAGE_ID=$(./bin/ore \
  hetzner \
  --hetzner-token $(pass hetzner/token) \
  --hetzner-location fsn1 \
  create-image \
  --name flatcar-nightly \
  --file http://bincache.flatcar-linux.net/images/amd64/4011.0.0+nightly-20240624-2100/flatcar_production_hetzner_image.bin.bz2)

./bin/kola \
  --platform hetzner \
  --hetzner-token $(pass hetzner/token) \
  --hetzner-server-type cx22 \
  --hetzner-location fsn1 \
  --hetzner-image "${HETZNER_IMAGE_ID}"
  run '*'

[tormath1]

Ingested in #536