Provide support for custom network and subnetwork in circleci-server-windows-image-builder script

[nanophate]

Description

Currently in the circleci-server-windows-image-builder, the default network and subnetwork are hard-coded and cannot be customized directly.

Users have reported the need to use a specific network and subnetwork, other than the default ones when configuring the circleci-server-windows-image-builder script.

script:
  project_id: '{{ user "project_id" }}'
  account_file: '{{ user "account_file" }}'
  zone: '{{ user "gce_zone" }}'
  subnetwork: default
  tags:
    - packer-winrm

https://github.com/CircleCI-Public/circleci-server-windows-image-builder/blob/feb9f7dc307fabb9317148e5138ee0fffcf53b25/windows/visual-studio/packer.yaml#L73C5-L73C15

Problem arises when users need to specify a custom subnetwork, as the current script does not support this feature.

Users can currently get around this issue by forking the script and changing the subnetwork value. However, having this feature directly in the script would enhance convenience and productivity.

Desired Outcome:

Support for custom network and subnetwork specification in circleci-server-windows-image-builder by making the subnetwork field dynamic in the script.

AC:

• Add option to specify custom network and subnetwork in the window/visual-studio/packer.yaml file
• Update the project documentation to reflect this change

related links:

circleci-server-windows-image-builder script on GitHub here
Hashicorp Packer Google Compute Builder documentation