[Feature]: Support write-only user_data on hcloud_server

[googol]

What whould you like to see?

The hcloud_server resource currently has the user_data attribute for applying cloud-init configs to the server on creation time. While not ideal, this string field might contain some sensitive information, so it is not desirable to have the contents stored in the terraform state. The new write-only arguments feature would make perfect sense here: only use the value of the user_data string on initial creation and then ignore it. Since the contents of user_data are only used when creating a server, there is no need to add a separate versioning attribute, users can be pointed to the standard replace_triggered_by lifecycle attribute.