feat: Allow publishing of RDS Postgres logs to Cloudwatch

What does this MR do?

The changes in this code update the configuration for RDS instances (relational database service) used in the GitLab reference architecture on AWS. Specifically, it adds the ability to enable CloudWatch logs exports and specifies a monitoring role ARN for Postgres instances. These changes enhance the monitoring and logging capabilities of the RDS instances, providing better visibility and control over database operations.

Related issues

closes https://gitlab.com/gitlab-org/gitlab-environment-toolkit/-/issues/857

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
  • 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 Quality Enablement team if unsure.