See L140-L156 in KEP: NOTE that NAD itself is a JSON, no change is needed there, we only need this change to OVNK's validation of the json config blob.

Goal of this card:

1. add support for parsing "primaryNetwork" field in https://github.com/ovn-org/ovn-kubernetes/blob/master/go-controller/pkg/util/multi_network.go
2. add tests for correct/wrong parsing in https://github.com/ovn-org/ovn-kubernetes/blob/master/go-controller/pkg/util/multi_network_test.go
3. add logic for existing NADs for secondary networks during upgrades to update this field to "false" - actually this might not be necessary, this key won't exist for secondary networks is my guess, I'll leave this to the card owner to figure out
4. either ways we need validation to ensure secondary network NADs can't have this field set to true etc

You don't need to start using the field yet, the changes can land independently since tests will ensure deadcode linter doesn't get tagged.