v5.1.0

Add compatibility to RKE2 cluster creation via Rancher, by:

• Allowing to reference existing SSH keys by ID, without needing to provide the original key file
• De-deprecating the hetzner-user-data-from-file flag, to support the Rancher default flow of injecting the user data file path into the user-data flag
• Introducing the hetzner-additional-user-data that performs an ordered YAML-merge with the Rancher-provided configuration file and additionally provided inline user-data