Allow to generate sidecar files from exiftool for non-supported filetypes

[Bionus]

There are a few reasons one might want to use sidecar files instead of embedded metadata:

• File types not supporting embedded metadata via exiftool (ex: webm)
• Not modifying files in-place, to keep the MD5 of the file intact

And while Grabber does support generating sidecar files and as such one could use the built-in system, it can make more sense to factorize this and rely of exiftool for this.

• https://exiftool.org/metafiles.html
• https://en.wikipedia.org/wiki/Extensible_Metadata_Platform

[ProtagNeptune]

https://docs.digikam.org/en/setup_application/metadata_settings.html#sidecars-settings

Filename for the sidecars set to filename.ext.xmp. For example, image1.dng will have a sidecar file named image1.dng.xmp. With the option Sidecar file names are compatible with commercial programs digiKam will create the XMP sidecar files with a compatible file name (image1.xmp) used by many commercial programs.