Implement EVPN for VXLAN-over-IPv6

[ipspace]

This commit introduces EVPN support for VXLAN-over-IPv6. The EPVN module checks vxlan.transport and activates EVPN AF on transport session matching VXLAN transport protocol. The choice of the transport session is stored in neighbor.evpn attribute (which was previously a bool)

While multiple configuration templates (EOS, FRR, VyOS) use the neighbor.evpn attribute to enable EVPN AF, only Arista EOS seems to have a working EVPN implementation for VXLAN-over-IPv6; the EVPN module thus checks evpn.ipv6 feature flag.