Merge pull request #12114 from opensourcerouting/feature/bgp_aigp_attribute

bgpd: Implement AIGP

diff --cc bgpd/bgp_route.c
Simple merge

diff --cc bgpd/bgp_updgrp_packet.c
Simple merge

diff --cc bgpd/bgp_vty.c
Simple merge

diff --cc bgpd/bgpd.c
Simple merge

diff --cc bgpd/bgpd.h
Simple merge

diff --cc doc/user/bgp.rst
Simple merge