Refactor subnets mgmt in net-vpc-factory
This commit removes the ability to define subnets inline within the VPC `.config.yaml` across `net-vpc-factory` and enforces file-based subnet definitions using the `subnets/` subdirectory pattern.
Key changes include:
- **Module Updates:** Removed standard and non-standard inline subnet arguments (`subnets`, `subnets_private_nat`, `subnets_proxy_only`, `subnets_psc`) from the factory module's internal `vpcs` instantiation.
- **Schema & Docs:** Renamed `vpc.schema.json` to `vpc-factory.schema.json`, stripping inline subnet definitions from the JSON schema, and updated the module `README.md` to reflect the new file-based only approach.
- **Stage 2 Networking Configs:** Refactored datasets to migrate inline `subnets_proxy_only` definitions into standalone files like `subnets/prod-proxy.yaml`. Added a new exported output `subnet_ips` for downstream usage.
<!--
**Breaking Changes**
```upgrade-note
`fast/stages/2-networking`: Proxy-only subnets have been renamed for consistency - unfortunately this results in a nasty create-before-destroy which needs to be handled manually (i.e. delete the existing proxy-only-subnet first, either manually or with a targeted apply, then apply again)
```
-->
This PR (re :D)introduces module net-vpc-factory, a minimal factory that deals with vpcs, subnets, and firewall rules creation, meant to be embedded into other factories, starting with 2-data-platform and 2-networking.
This PR renames `2-networking-foo` stages to `2-networking-legacy-foo` - including references of stages in docs and tests.
**Breaking Changes**
```upgrade-note
`fast/stages/2-networking-a-simple`: The stage is being deprecated, and superseded the `2-networking` FAST stage, introduced in #3435
`fast/stages/2-networking-b-nva`: The stage is being deprecated, and superseded the `2-networking` FAST stage, introduced in #3435
`fast/stages/2-networking-c-separate-envs`: The stage is being deprecated, and superseded the `2-networking` FAST stage, introduced in #3435
`modules/net-vpc-factory`: The module has been deprecated, and superseded the `2-networking` FAST stage, introduced in #3435
```
This pull request introduces the `net-vpc-factory` module. This new factory handles:
* Project setup (most of what's supported by the `project` module)
* VPC setup
* Routing
* Subnets
* Connectivity options
* NCC (hub, VPC spokes and VPN hybrid spokes)
* Peerings
* VPN (GCP-to-onprem and GCP-to-GCP)
* NAT (everything supported by the `net-cloudnat` module)
* DNS (everything supported by the `dns` module)
* Firewall (everything supported by the `net-vpc-firewall` module)
