feat: add supplemental cloud-init user-data support

[brngates98]

Summary

Addresses #68.

Adds optional supplemental cloud-init user-data configuration, similar to other Rancher node drivers (e.g. Harvester/VMware). Users can start from stock cloud-image templates and layer custom cloud-config at node pool creation time instead of baking everything into a custom template.

Changes

• Add --pve-cloud-init / PVE_CLOUD_INIT for an optional user-data source (local file path or URL)
• Add --pve-cloud-config / PVE_CLOUD_CONFIG for an optional cloud-config YAML file path
• Merge user-provided cloud-config with driver defaults (hostname, SSH user, Rancher SSH public key injection)
• Expose both options in the Rancher UI extension
• Add unit tests for default generation, cloud-config merge, and file/URL loading

If both options are set, --pve-cloud-config takes precedence over --pve-cloud-init.

Example cloud-config

For the use case in #68, users can supply something like:

#cloud-config
packages:
  - qemu-guest-agent
  - nfs-common
runcmd:
  - systemctl enable --now qemu-guest-agent

This does not auto-install qemu-guest-agent in the default generated user-data. Users supply packages and bootstrap steps via the new options above.

Test plan

• go test ./cmd/docker-machine-driver-pve/driver/ -run 'TestGenerateCloudinit|TestLoadCloudConfig'
• Create a VM from a stock cloud-image template with --pve-cloud-config pointing at a YAML file that installs qemu-guest-agent
• Verify cloud-init completes successfully
• Verify SSH access works with the injected Rancher public key
• Verify qemu-guest-agent is installed and running
• Verify --pve-cloud-init works with a local file path
• Verify --pve-cloud-init works with a URL
• Verify the new Cloud-init fields appear in the Rancher UI extension and persist to machine config

Related

• Unable to customize user-data to do things like install qemu-guest-agent #68