ovn-northd: Add logical flows to support native IPv6 RA

ovn-northd: Add logical flows to support native IPv6 RA

This patch adds logical flows which sends IPv6 Router Advertisement
packet in response to the IPv6 Router Solicitation request. It uses
the actions "put_nd_ra_opts" to transform the RS packet to RA packet
in the newly added ingress stage "lr_in_nd_ra_options" in router
pipeline. If the action "put_nd_ra_opts" is successful, it sends the
RA packet back to the originating port in the next ingress stage
"lr_in_nd_ra_response".

A new column "ipv6_ra_configs" is added in the Logical_Router_Port
table, which the CMS is expected to configure IPv6 RA
configurations - "address_mode" and "mtu" for adding these flows.

Co-authored-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Zongkai LI <zealokii@gmail.com>
Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Acked-by: Miguel Angel Ajo <majopela@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>