Schedule generation of an export (JSON, PDF or Excel) for a single report.

Export generation is asynchronous: this endpoint enqueues the work and
immediately returns a generation_id. Poll
GET /external/export/status?generation_id={generation_id} until the response
reports is_finished: true; the finished payload then carries the JSON export
data (for JSON exports) or a pre-signed download URL (for PDF / Excel exports).

Choosing which export to run

• Omit both export_id and export_name to run your default report export.
• Provide export_id to run a specific export by its identifier, or
  export_name to run it by name. Provide at most one of the two; sending
  both returns 400.

The output format is determined by the export's own definition.