Add the ability to provide custom metadata for GCP instances

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This MR adds the ability to provide custom metadata for the GCP instances provisioned by the GET.

There are 2 flavors of variables:

1. custom_metadata: Metadata added here will be applied to all GCP instances provisioned by the GET.
2. <component>_custom_metadata: Metadata added here will only be applied to the GCP instances for the given component. For example, gitaly_custom_metadata will only be applied to the gitaly instances.

if both custom_metadata and <component>_custom_metadata are used and contain the same metadata key, then the <component>_custom_metadata takes precedence for the given component.

closes #1002 (closed)

• 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
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
  • 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.
    • If this MR adds an optional configuration - check that all permutations continue to work.
    • For Terraform changes: set up 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.
• 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 Self-Managed Platform team if unsure.