[feature] allow users to specify preview image

[whatdoineed2do]

I have some, what appears to be broken DNG files, that were created with no previews, although the raw data is fine and loads in various raw processors.

I would like to regenerate a new DNG but embedding a preview image from command line, something like:

dnglab convert -dng-preview-image preview.jpg orig.dng new.dng

Using existing functionaly of 0.5.2, I can dnglab convert -dng-preview true orig.dng new.dng which generates a file that has a very small preview image, even if exiftool believes the stripbytecount (tag 0x0117) is too undersized (original dng does not have this tag)

[cytrinox]

This is possible with makedng command from main branch, but that's very low level.

Related to #371

[whatdoineed2do]

Ahead of any new higher level implementation, would it be possible to give examples of how to use both the extract and makedng commands?

A quick look at the source code with -map options makes me nervous that I won't know all of the items to extract to reconstruct