Export AWS subnet ids as outputs

What does this MR do?

Closes #390 (closed) by exporting the subnet ids to avoid fragility of using data sources in GET-wrapping terraform that needs those IDs to create further resources.

Related issues

#390 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
    • No existing documentation of outputs, not sure where would be best to add any
  • If this MR adds an optional configuration - check that all permutations continue to work.
    • N/A
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
    • N/A, only output changes
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.