Add IPv6 address support to linknets

[lunkwill42]

Fixes #263

At Sikt, we want to run the underlay directly on IPv6. This requires support for IPv6 network address fields on the Linknet model, which must also be supported by the API.

This is still in draft mode, currently awaiting two things:

1. We also need to add an infra_ipv6 field to Devices, with the associated functionality. E.g., this is required to be able to populate bgp_ipv6_peers as a device variable, among other things.
2. We need to test this in our own lab deployment.

Whereas CNaaS-NMS requires an IPv4 linknet to be /31, this PR will accept IPv6 linknets as large as /64, as this is standard practice in our case. Still, only the two first host addresses are picked from a linknet of this size.