* feat(fast): add attachment groups factory to 2-networking
Adds support for `google_compute_interconnect_attachment_group` in the `2-networking` stage.
By implementing this at the factory level alongside `vlan-attachments`, users can now declaratively aggregate VLAN attachments across multiple VPCs and natively reference them using the `$attachment_groups:<key>` context identifier in their configuration YAMLs.
Includes:
- Factory implementation in `factory-vlan-attachments.tf`.
- New JSON schemas for `attachment-groups` and updates to `vlan-attachments` to support context linkage.
- Test coverage with new inventory generations.
Fixes#3791
9eb69ffaa3