feat: allow specifying custom common omnibus config

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What does this MR do?

This MR allows users to configure custom gitlab config across all omnibus types. This is done similar to existing custom per-component configs, and the logic is inserted into the common role.

This is to reduce code reuse in some circumstances. For instance, in the firm we have all hosts bootstrapped with a consul agent upon build -- to set up gitlab we have to specify custom consul ports within gitlab.rb to de-conflict between our firm's consul cluster and the omnibus's consul cluster.

In this case, it makes sense to have common custom config that can be used to set the custom consul ports once which will be applied uniformly to all nodes.

Related issues

Closes #868 (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.