Add object storage settings for GitLab Pages

What does this Merge Request do?

In gitlab!42461 (diffs) we add support for object storage settings in pages.

It doesn't break without these settings, but tests won't pass without changing gitlab.yml locally(in CI we use example file), this MR.

I used terraform_state settings as an example - !1158 (diffs).

Not sure if we need tests for this 🤷

part of gitlab#245295 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.