Support direct network attachment

[zoro16]

This allows attaching a server resource to a network (private) on creation. This will allow to set a specific private IP address to the server along with optional list of alias_ips.

resource "hcloud_server" "server" {
  ...
  network {
    network_id = var.private_network_id
    ip         = var.private_network_ip
    alias_ips  = var.private_network_alias_ips
  }
}

[zoro16]

This PR is included in version 1.1.0 🎉