feat(plugin-import-export): preview displays CSV and JSON data accurately

[PatrikKozak]

What

This PR updates the import-export plugin's <Preview /> component to render table columns and rows using the same logic as the CSV export.

Key changes:

• Adds a new /api/preview-data custom REST endpoint that:
  • Accepts filters (fields, where, sort, draft, limit)
  • Uses getCustomFieldFunctions and flattenObject to transform documents
  • Returns deeply flattened rows identical to the CSV export
• Refactors the component to:
  • POST preview config to the new endpoint instead of querying the collection directly
  • Match column ordering and flattening logic with the createExport function
• Ensures consistency across CSV downloads and in-admin previews
  -Adds JSON preview

This ensures preview results now exactly match exported CSV content, including support for custom field transformers and polymorphic fields.

[DanRibbens]

This needs to hit a new endpoint so that the table data can include changes from toCSV function calls.