Implemented via the same mechanism as for image_download_source=http.
Forcing raw format (and thus streaming) is supported as well.
Since we always re-calculate checksums for such images, the requirement
on providing them via the API is lifted.
Change-Id: Ife385c3b363c28559c90e5a54e9c6f6807d207ff
Story: #2008075
Task: #40764