This card tracks the major chunk of the foundational work to support user defined primary network in OVNK of type "L2" -> use the existing L2 secondary controller logic here to change our default network controller to make the necessary changes

Goal of the card is to Implement the following workflow:
 * if feature gate is enabled take action on NAD events created with "primaryNetwork:true"
 * validate namespace doesn't have existing pods - if so, error out and emit event to warn user -> this logic will be same on card https://issues.redhat.com/browse/SDN-4937 so work with that assignee on common utils
 * validate namespace doesn't have more than 1 NAD - if so error out
 * validate all fields in the NAD
 * finally if all is well annotate the namespace with "k8s.ovn.org/active-network: l2-network" ;
 * then check if this same "named" NAD annotation is already present on other namespaces; if so - the topology already exists; nothing to do
 *  note that topology for pod2pod is already done for secondary networks so leverage logic from there
 * DO NOT WORRY ABOUT NORTH-SOUTH logic in this card. There will be follow-up card for that
 ** Add e2e tests for pod east-west on user-defined network - meaning run existing conformance for pods on the new network*
 ** NOTE: This traffic flow is the same for both gateway modes, so make sure to test on both*

{}End goal here is: Only add support for east-west on user-defined network for the pods. Other things will have other cards. Please work with other assignee as a "pod-design-sub-group" so that you also have your potential reviewer in your group.{}