Add agent_plan_content object storage configuration

What does this MR do?

Adds the agent_plan_content bucket configuration to our charts.

Related issues

• Create WorkItem::AgentPlan and its table, with ... (gitlab-org/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/omnibus-gitlab!9382)

Author checklist

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Required

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

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.