Add first iteration of GitLab Pages support

What does this MR do?

This MR adds support for serving GitLab Pages from separate nodes.

In order to achieve this there is new role gitlab_pages added. The roles for rails and sidekiq have adjusted accordingly. Along this changes new variables have been added, where the user has to set at least external_pages_url. This MR has only been tested with object storage.

There isn't yet any corresponding terraform code for this. The amount of nodes is arbitrary but has to be at least one to use this.

Related issues

#408 (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.