IPv6 in linknets

[vidarstokke]

Is your feature request related to a problem? Please describe.
We need the ability to run IPv6 in linknets. Currently it is not possible to add IPv4 addresses in linknets.

Describe the solution you'd like
It needs to be possible to add these settings to linknets:

• ipv6_network
• device_a_ipv6
• device_b_ipv6

Describe alternatives you've considered
We've considered deriving IPv6 address from the IPv4 address. But this is not optimal for several reasons. Most of all it is not possible if we want to go for a IPv6 only underlay.

[indy-independence]

This one has very low priority from sunet side, basically because underlay addresses will not be seen by clients, not seen in traceroutes, not used by any management/monitoring systems etc so we don't really se the point of doing IPv6 here. Also Arista didn't support doing VXLAN/EVPN overlay on top of IPv6 only underlay until very recent releases. But as long as it doesn't add too much complexity we can merge some PRs! :)