Add support for software defined networking (SDN)

[vmj]

There's a new Network resource, part of which is a new IpNetwork resource. One can list, create, modify, and delete these networks.

• https://developers.upcloud.com/1.3/13-networks/#list-all-networks
• https://developers.upcloud.com/1.3/13-networks/#list-networks-within-zone
• https://developers.upcloud.com/1.3/13-networks/#create-sdn-private-network
• https://developers.upcloud.com/1.3/13-networks/#get-network-details
• https://developers.upcloud.com/1.3/13-networks/#modify-network-details
• https://developers.upcloud.com/1.3/13-networks/#delete-network

When dealing with Server resources, there's a new list of Interface resources in the networking property.

• https://developers.upcloud.com/1.3/8-servers/#get-server-details
• https://developers.upcloud.com/1.3/8-servers/#create-server

These Interface resources can also be created after the server creation. The server must be stopped and the network must already exist.

• https://developers.upcloud.com/1.3/13-networks/#create-network-interface
• https://developers.upcloud.com/1.3/13-networks/#modify-network-interface
• https://developers.upcloud.com/1.3/13-networks/#delete-network-interface

[vmj]

Also, add networks to the billing summary.