Improve auto-direct-node-routes so it doesn't exit when a node is outside of the current subnet.

Is your proposed feature related to a problem?

When running Cilium in native routing mode with the BGP Control Plane in a multi-zone setup, it is not possible to have same-zone traffic routed over L2 as each node is not aware of each others pod CIDR so it needs to use its default gateway, bottlenecking same-zone traffic.

Since we have multiple zones that have different subnets, it's not currently possible to enable auto-direct-node-routes as it will fatally exit when it detects the nodes which have different subnets.

Describe the feature you'd like

Improve auto-direct-node-routes so it ignores nodes that are outside of the current subnet, only adding routes for the nodes that are in the current subnet.

For example, if I have 8 nodes in zone A and 8 nodes in zone B, all zone A nodes will have direct node routes to all nodes in zone A and all zone B nodes will have direct node routes to all nodes in zone B.

More discussion here: #31091