Add namespace_traversal_ids to issues

What does this MR do and why?

Add namespace_traversal_ids to issues

Adds namespace_traversal_ids to the issues table to prepare an optimization for querying namespaces within hierarchies.

Changelog: added

References

Part of the effort &18644 (closed)

For more information about the plan, see https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/traversal_ids_on_issues/

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