zebra, bgp: Support type-5 routes with asymmetric routing

zebra, bgp: Support type-5 routes with asymmetric routing

Asymmetric routing is an ideal choice when all VLANs are cfged on all leafs.
It simplifies the routing configuration and
eliminates potential need for advertising subnet routes.
However, we need to reach the Internet or global destinations
or to do subnet-based routing between PODs or DCs.
This requires EVPN type-5 routes but those routes require L3 VNI configuration.

This task is to support EVPN type-5 routes for prefix-based routing in
conjunction with asymmetric routing within the POD/DC.
It is done by providing an option to use the L3 VNI only for prefix routes,
so that type-2 routes (host routes) will only use the L2 VNI.

Signed-off-by: Mitesh Kanjariya <mitesh@cumulusnetworks.com>