Provide support for Excel Export with custom aggreagte in Ej2 Treegrid Scheduled

To enable support for custom aggregates during Excel export in Syncfusion EJ2 TreeGrid, the TreeGrid's export module should be enhanced to evaluate and include user-defined aggregate functions in the exported data. This would involve extending the current export pipeline to recognize and process custom aggregate definitions provided in the TreeGrid configuration. The system should invoke the custom aggregate logic during export, just as it does when rendering the grid, and map the resulting values to the corresponding footer or group summary cells in the Excel sheet. Additionally, template rendering used for custom aggregates should be interpreted and converted into plain text or Excel-compatible formatting for accurate export representation. This built-in support would eliminate the need for manual workarounds using beforeExcelExport or excelQueryCellInfo, greatly simplifying the developer experience and ensuring consistency between the on-screen data and exported files.