feat(gcp): Add Hierarchical Namespace (HNS) support for GCS bucket

[varunarya002]

Issue:
GCS buckets with HNS enabled require explicit folder creation and storage.folderAdmin permissions for managed folder operations. Without this, Iceberg table creation and Spark ingestion fail on HNS buckets.

Changes:

• Add hierarchicalNamespace boolean field to GcpStorageConfigInfo (OpenAPI spec)
• Add @nullable Boolean isHierarchicalNamespace() to GcpStorageConfigurationInfo
• Generate storage.folderAdmin access boundary rule scoped to managedFolders/{path} for write locations when HNS is enabled
• Plumb HNS flag through CatalogEntity model<->entity conversion
• Add backward-compatible 3-param overload defaulting HNS to false

Tests:

• HNS enabled single bucket (read + write + folder rules)
• HNS with multiple buckets and partial writes
• HNS without writes (no folder rules)
• HNS with separate metadata and data buckets
• HNS same bucket with separate metadata/data paths
• Non-HNS vs HNS rule count comparison
• Config serialization round-trip
• CatalogEntity round-trip with HNS flag

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

[dimas-b]

Thanks for your contribution, @varunarya002 !

Since this PR affects REST API parameters, please open a corresponding discussion on the dev ML, which is a usual practice in Polaris.

(this is not a complete review from my side 😅 ... just noting a couple of points for a start)

[dimas-b]

nit: a similar field in Azure is called simply hierarchical