Add agent_plan_content config and MinIO bucket

What does this merge request do and why?

Adds agent_plan_content storage config templating to GDK, and creates its bucket when setting up MinIO. Closes Create agent-plan-content bucket in GDK (gitlab#600508 - closed).

See:

• https://gitlab.com/gitlab-org/gitlab/-/work_items/596369+
• Create WorkItem::AgentPlan and its table, with ... (gitlab!232335 - merged)
• Add agent_plan_content object storage configura... (gitlab-org/charts/gitlab!5029 - merged)
• Add agent_plan_content object storage configura... (omnibus-gitlab!9382 - merged)

How to set up and validate locally

1. Check out the branch.
2. gdk config set object_store.enabled true && gdk reconfigure
3. Check the generated gitlab/config/gitlab.yml; it should contain an agent_plan_content stanza.
4. Check at http://gdk.test:9002/ (or wherever your MinIO console is) (login minio / gdk-minio) that the agent-plan-content bucket is created.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.