Render Gitaly pack_objects_cache settings

What does this MR do?

This makes the Gitaly pack-objects cache configurations settings, introduced in gitaly!3264 (merged), configurable.

This settings is off by default in attributes/default.rb.

Related issues

gitlab-com/gl-infra/scalability#953 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened