Add top/host level configuration for airgap

Instead of using the `files:` feature, it would work like the k0s binary uploads.

```yaml
spec:
hosts:
- role: controller
airgap: true
```

```yaml
spec:
hosts:
- role: controller
airgapBundlePath: files/k0s-airgap-bundle-x86-1.23+k0s.4 # similar to the k0sBinaryPath
```

This could also have a feature that would prepare your k0sctl host's caches, something like `k0sctl prepare-airgap` which would download all the required binaries + bundles to local cache, then you can take your machine to the bunker and run `apply` there. Maybe with something like `--output-directory /mnt/usb_stick/k0s_binaries`.

Trace the existing `files:` feature and k0s binary upload handling, especially the `k0sBinaryPath` path configuration. Then inspect the `apply` flow and the proposed `k0sctl prepare-airgap` entry point to define the scope for host-level airgap settings and cache preparation. Done means the requested configuration and offline preparation workflow are supported without relying on ad hoc file entries.

Written by the indexing model from the issue text.