Support adding vSwitch subnets to the network

[dannynodies]

/kind feature

Describe the solution you'd like
At present, if I add a vSwitch subnet to the created network, it causes NetworkReconcileFailed : failed to find network: multiple subnets not allowed

I am attempting to route traffic between the cluster and some baremetal servers over the private network, and allowing the addition of a vSwitch subnet would permit this configuration to be applied.

Related to #1472 (since the changes required would conflict)

Anything else you would like to add:
I have a pretty good idea how this can be done - will submit a PR

Environment:

• cluster-api-provider-hetzner version: v1.0.0
• Kubernetes version: 1.30.5
• OS (e.g. from /etc/os-release): Ubuntu 22.04