[Feature] Add Linode support for networking and user data

[github-actions]

This is the tracking issue for #2121

Please add labels indicating the release versions eg. 'release/v14'

Please add comments for user issues which this issue addresses.

Description copied from PR:
Fixes #2120

Description

Adds support for the new Linode machine config fields in rancher2_machine_config_v2 so Terraform can configure the Linode machine driver updates for:

• interface/VPC networking
• metadata user_data / cloud-init

This change updates the Linode machine config schema, expand/flatten handling, validation, docs, and tests.

New linode_config fields:

• use_interfaces
• vpc_subnet_id
• vpc_private_ip
• public_interface_firewall_id
• vpc_interface_firewall_id
• user_data

Compatibility notes:

• Existing legacy create_private_ip support is preserved.
• Validation prevents mixing legacy private IP mode with interface/VPC mode.
• Validation also enforces vpc_subnet_id when use_interfaces is enabled.

This PR depends on the corresponding Linode machine driver support landing first:

• feat: add VPC interface networking and firewall controls linode/docker-machine-driver-linode#220
• feat: add cloud-init user data support via Linode Metadata linode/docker-machine-driver-linode#222

Until those changes are merged and available in the driver consumed by Rancher, these provider fields will not be usable end-to-end.

Testing

Updated unit tests cover Linode machine config expand/flatten behavior for the new fields.

I did not run the full provider test suite as part of opening this PR.

Not a breaking change.