bgpd: Handle connected routes and IPv6 link-local nexthops for route leaking

bgpd: Handle connected routes and IPv6 link-local nexthops for route leaking

Connected routes redistributed into BGP as well as IPv4 routes with IPv6
link-local next hops (RFC 5549) need information about the associated
interface in BGP if they are candidates to be leaked into another VRF. In
the absence of route leaking, this was not necessary. Introduce the
appropriate mechanism and ensure this is used during route install (in
the target VRF).

Ticket: CM-20343, CM-20382
Testing done:
1. Manually verified failed scenarios and some additional ones - logs
in the tickets.
2. Ran bgp-min and evpn-min - results are good.
3. Ran vrf smoke - has some failures, but none which look new

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>