Allow config to use BOM when export csv for "import export plugin"

[arku3]

MS Excel need the BOM (Byte Order Mark) for UTF-8 encoded CSV to open correctly.

https://support.microsoft.com/en-us/office/opening-csv-utf-8-files-correctly-in-excel-8a935af5-3416-4edd-ba7e-3dfd2bc4a032

In my use case, we have a form setup and need to download submissions as csv for follow up actions offline. As our native language isn't english, so when a layman user double click the csv to open with MS Excel, it looks broken. While there is workaround and use data import in excel, it would be nice if we can choose to create the CSV with BOM.