Support offline installation of helm chart

What does this MR do?

Following the implementation steps from the issue below, I have done the following:

• Added the rule to not run when offline_setup is true for kubernetes.core.helm_repository calls for both Add and update charts
• Added gitlab_charts_ref to configure a custom charts_ref for the kubernetes.core.helm calls and defaults to "{{ gitlab_charts_repo_name }}/gitlab"
• Also added kubernetes.core.helm not run hen offline_setup is true with the exception of when gitlab_charts_ref has been changed from the default (indicating custom usage)
• Added docs in the offline setup section detailing the above

Thanks to @nprabakaran for getting the ball rolling on this one.

Related issues

Closes #726

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.

Closes #726