bgpd: support evpn nd ext community

bgpd: support evpn nd ext community

EVPN ND ext community support NA flag R-bit, to have proxy ND.

Set R-bit in EVPN NA if a given router is default gateway or there is a
local
router attached, which can be determine based on local neighbor entry.

Implement BGP ext community attribute to generate and parse  R-bit and
pass along zebra to program neigh entry in kernel.

Upon receiving MAC/IP update with community type 0x06 and sub_type 0x08,
pass the R-bit to zebra to program neigh entry.

Set NTF_ROUTER in neigh entry and inform kernel to do proxy NA for EVPN.

Ref:
https://tools.ietf.org/html/draft-ietf-bess-evpn-na-flags-01

Ticket:CM-21712, CM-21711
Reviewed By:
Testing Done:
Configure Local vni enabled L3 Gateway, which would act as router,
checked
show evpn arp-cache vni x ip <ip of svi> on originated and remote VTEPs.
"Router" flag is set.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>