Add ability to pass in Custom GitLab secrets

What does this MR do?

MR expands on the work done in !1027 (merged), !1140 (merged) and !1181 (merged) to allow Custom GitLab secrets to be passed in by the user for better direct control of the secrets.

With this MR users can now pass in some or all GitLab secrets via a JSON string that the Toolkit will merge in accordingly with precedence across the environment.

Some refactoring was required for Geo to ensure a separation of concerns for the specific path of passing a non GET controlled Primary's full secrets to a GET controlled secondary. To achieve this variables previously named with the prefix gitlab_custom_* in !1140 (merged) have been renamed to geo_primary_* (A non breaking change as the last MR was in this dev cycle).

Related issues

Closes #798 (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 Quality Enablement team if unsure.