Allow use of any configured file system scheme

Problem/Motivation

The data export supports just the two default file system schemes public:// and private://

There are many contrib modules which register additional file system schemes (for example saving to AWS S3 and Dropbox), and drupal allows configuration of the default value for this which can be set at /admin/config/media/file-system and is saved to the config item 'system.file' as 'default_scheme'.

Views Data Export doesn't recognise these additional filesystems, however, and won't allow exports to be saved to these.

Proposed resolution

Instead of choosing public or private, perhaps we could instead use the same selection mechanism for file system wrappers as used by core:

$config = $this->config('system.file');
$form['file_default_scheme'] = [
  '#type' => 'radios',
  '#title' => t('Default download method'),
  '#default_value' => $config->get('default_scheme'),
  '#options' => $options,
  '#description' => t('This setting is used as the preferred download method. The use of public files is more efficient, but does not provide any access control.'),
];