iproute2: add 'ip monitor acaddress' support

iproute2: add 'ip monitor acaddress' support

Enhanced the 'ip monitor' command to track changes in IPv6
anycast addresses. This update allows the command to listen for
events related to anycast address additions and deletions by
registering to the newly introduced RTNLGRP_IPV6_ACADDR netlink group.

This patch depends on the kernel patch that adds RTNLGRP_IPV6_ACADDR
being merged first.

Here is an example usage:

root@uml-x86-64:/# ip monitor acaddress
2: if2    inet6 any 2001:db8:7b:0:528e:a53a:9224:c9c5 scope global
       valid_lft forever preferred_lft forever
Deleted 2: if2    inet6 any 2001:db8:7b:0:528e:a53a:9224:c9c5 scope global
       valid_lft forever preferred_lft forever

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Signed-off-by: David Ahern <dsahern@kernel.org>