bgpd: encode implicit next-hop-self in some vrf route leaking scenarios

bgpd: encode implicit next-hop-self in some vrf route leaking scenarios

Problem reported that when vrf route-leaking between an unnumbered
peer in one vrf to a numbered peer in another vrf, the nexthop
attribute was missing from the update, causing the session to fail.
determined that we needed to expand the  mechanism for verifying if
the route has been learned in the other vrf without an ipv4 nexthop.

Ticket: CM-25610
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>