Add support for private cloud (the hosts APIs)

[vmj]

• A new resource Host, which has (among other simple properties) a List<Stat> stats property
• A new method UpCloudApi::hosts that returns a list of List<Host> hosts resources
• A new API trait HostApi with the methods:
  • HostApi::load(...args)
  • HostApi::update(Resource, ...args) (only Host::description seems to be allowed here`)

Question: the properties of Host or Stat are not documented?

https://developers.upcloud.com/1.3/14-hosts/