Added feature to specify a custom Auto DevOps chart repository

What does this MR do?

This MR adds the project variable AUTO_DEVOPS_CHART_REPOSITORY which defaults to https://charts.gitlab.io to the Auto DevOps template.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#55791

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer