Re-add missing sftp:// stream wrapper

Fixes #9320
Regression from #8729 (entirely my fault apparently)

Without this patch, SFTP external storage is not fully usable. As such this should be backported to stable7. stable6 (and possibly earlier) is not affected.

File inclusion has been added to the constructor such that this is not yet another side effect.

The following manual tests have been performed via the webinterface:
* Deletion of existing file in subfolder
* Deletion of existing empty directory
* Deletion of existing directory containing files
* Mass deletion of directories containing files
* Upload of a file
* Download of a file
* Detection of file details (filesize etc.)

Futhermore upload and download of files via WebDAV has been tested.

@PVince81 @DeepDiver1975