bgpd: changes for maintaining evpn nexthops and their rmac mapping

bgpd: changes for maintaining evpn nexthops and their rmac mapping

In the case of EVPN type-2 routes that use ES as destination, BGP
consolidates the nh (and nh->rmac mapping) and sends it to zebra as
a nexthop add.

This nexthop is the EVPN remote PE and is created by reference of
VRF IPvx unicast paths imported from EVPN Type-2 routes.

zebra uses this nexthop for setting up a remote neigh enty for the PE
and a remote fdb entry for the PE's RMAC.

Ticket: CM-31398

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>