partclone.dd: please support the sparse file format to allow mounting images without transferring empty areas

[KaiHerlemann]

There were already FRs that in favor of supporting the sparse file format. But the solution that was recommended is not suitable for our use case.
We would like be able to open images with 7-Zip or mount them with the Linux mount command to extract a few files without restoring the entire image.
This is possible with partclone.dd, not with partclone.[fs] or partclone.imager.

However, when using partclone.dd, the copy process is much slower. In contrast, when a sparse image is created with dd, empty areas aren't saved (and, of course, aren't transferred).