It's not obvious that a load balancer is required for HA

### Is your feature request related to a problem? Please describe.

I know it's documented, but when I read the docs the first time I missed the nuance about control plane connectivity issues without having a tcp load balancer and spent quite a while debugging konnectivity-agent "duplicate server" errors.

### Describe the solution you would like

It would be awesome if k0sctl required either `spec.api.externalAddress` or `spec.network.nodeLocalLoadBalancing` to be set when there are multiple controllers defined for the cluster. If there are valid deployments without a load balancer then maybe a warning during `apply` would be sufficient.

### Describe alternatives you've considered

_No response_

### Additional context

None, just wanted to say thanks for what is a great project!

Start at the apply entry point and trace how multiple controllers, spec.api.externalAddress, and spec.network.nodeLocalLoadBalancing are represented and validated. Done means apply clearly rejects or warns when a multi-controller cluster has neither setting, while preserving valid deployments that do not need a load balancer.

Written by the indexing model from the issue text.