upload: add a the possibility to pass a custom description for the uploads

Fossology has a "description" field for every upload, that currently is filled by a4f with a standard hard-coded text ('uploaded by aliens4friends').

It would be very useful to add an option to set a custom description via CLI (eg. one may want to use 'uploaded from pipeline #XXXXXX project OSTC/OHOS/manifest MR #X') in order to be able to identify, in Fossology, which uploads have been uploaded by which pipeline in consequence of which Merge Request.