For primary UDN layer2 the proper IPv6 gateway that should be configured
with received router advertisements is the one from the local GR IPv6
LLA, this change add field to the ovn pod annotations with that
information so it can be consumed by follow up PRs that will ensure
remove GRs are ignore as layer2 ipv6 gateways.

https://github.com/ovn-kubernetes/ovn-kubernetes/pull/4866

continuing that on https://github.com/ovn-kubernetes/ovn-kubernetes/pull/4878