Allow easily fetching the public IPv6 address of a Server

[elnygren]

The Server.get_public_ip method at https://github.com/UpCloudLtd/upcloud-python-api/blob/master/upcloud_api/server.py#L331 currently prefers IPv4 addresses making fetching a public IPv6 address hard.

This is a concern especially for the upcloud-ansible inventory module.

[elnygren]

ec16d0c