Add ability to pass through certificate values for NGinx SSL (Single Node)

What does this MR do?

The merge request updates the configuration of GitLab Rails NGINX SSL certificates and keys. It introduces new variables to allow specifying the certificate and key directly as strings, instead of relying on local files. This enables retrieving the certificate and key dynamically, for example, through a lookup mechanism. Additionally, the documentation is updated to reflect these changes and provide an example configuration.

Related issues

Closes #947 (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
  • 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.