bgp: want extended next-hop support #580
Description
With MP-BGP support in place for IPv4 Unicast and IPv6 Unicast address-families, the foundation will be laid for consolidation of IPv4/IPv6 route exchange over a single IPv6 transport session using IPv6 next-hops.
The extended next-hop capability (RFC 8950, formerly RFC 5549) describes the facilities for encoding an NLRI from one address-family with a next-hop from a different address-family.
For our purposes, this would really only be used to enable IPv4 Unicast routes to be exchanged with IPv6 next-hops, since IPv6 routes can already encode IPv4 next-hops via IPv4-mapped IPv6 addresses (e.g. ffff::10.0.0.1).