Add agent_plan_content object storage configuration

What does this MR do?

Adds the agent_plan_content bucket configuration to Omnibus.

Related issues

• Create WorkItem::AgentPlan and its table, with ... (gitlab!232335 - merged)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/598408#note_3312519214 (re: using a separate bucket)
• Add agent_plan_content object storage configura... (gitlab-org/charts/gitlab!5029)

Checklist

See Definition of done.

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

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled ~"workflow::ready for review" per the Distribution MR workflow.
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• The UBT version and corresponding checksum hash have been updated and referenced in the merge request if applicable.
  • UBT EE pipeline (Trigger:ee-package-ubt) is green

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
  • To debug QA failures, refer to the Investigate QA failures section.
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• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
  • Validation is covered by the Chef spec changes; these assert the rendered gitlab.yml, the directory creation, and bucket override behaviour. The feature this backs (Create WorkItem::AgentPlan and its table, with ... (gitlab!232335 - merged)) is FF gated and has no production consumers yet, so end-to-end validation against a built package wouldn't exercise any user-facing path beyond what the specs already cover.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
  • No new e2e behaviour to exercise; Create WorkItem::AgentPlan and its table, with ... (gitlab!232335 - merged) wires agent_plan_content into Rails but has no user-facing consumer yet (and when it does, it's FF-gated). Existing object storage QA scenarios cover it, I think.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.