feat(helm): provision built-in Grafana dashboards

[GatewayJ]

Type of Change

• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Refactor
• Other:

Related Issues

Closes #252

Summary of Changes

• Vendor all seven Grafana dashboards from rustfs/rustfs revision 6ef0740afeb5ae3bc890774b8e83d7e21c94a7e3 into the Helm chart.
• Publish one ConfigMap per dashboard by default with configurable discovery labels and namespace.
• Validate dashboard JSON, unique UIDs, Prometheus data-source variables, and enabled/disabled/custom Helm rendering.
• Document the Grafana sidecar and Prometheus prerequisites.

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit (fmt-check + clippy + test + console-lint + console-fmt-check)
• Added/updated necessary tests
• Documentation updated (if needed)
• CHANGELOG.md updated under [Unreleased] (if user-visible change)
• CI/CD passed (if applicable)

Impact

• Breaking change (CRD/API compatibility)
• Requires doc/config/deployment update
• Other impact:

Verification

make pre-commit
helm lint deploy/rustfs-operator
helm package deploy/rustfs-operator --destination /tmp

Additional Notes

The chart publishes dashboard ConfigMaps but does not install Grafana or Prometheus. By default, the ConfigMaps use grafana_dashboard: "1" in the operator namespace; deployments whose Grafana sidecar watches another namespace can set dashboard.namespace.

---

Thank you for your contribution! Please ensure your PR follows the community standards (CODE_OF_CONDUCT.md) and sign the CLA if this is your first contribution.