Potentially avoid round-robin routing among default gateways

Presently, when a container is connected to multiple networks, Netavark adds each network's default route to the container's routing table. The kernel then automatically round-robins between these routes, as they are identical aside from their destinations.

However, this behavior may not be desirable for all users; having a single exit point for all traffic leaving the container could be desirable for writing firewall rules or similar.

We could only add a single default route, but this would make connecting and disconnecting networks much more difficult (we'd have to determine if a default was already present and create if not). Alternatively, we could use route priority to prefer one of the default gateways, but this runs into similar difficulties.