Extend debian-pipeline workflow with an upload to a staging repository

Before releasing an update, one might want to test it. And to test it, it can be helpful to have the package available in a "staging" repository.

The proposal is thus to extend the debian-pipeline workflow to make it possible to upload source and binaries, as soon as they are built into an external staging repository. The upload would likely not be signed since they are not yet validated.

Things to consider:

• this would require the use of a prefix for the staging "package-upload" workflow since we already have one such workflow
• we probably want to offer less flexibility for that staging upload workflow
  • do we allow the packages to be signed? probably, one could want to use a different key with less privileges...
  • we probably want to upload ASAP each .changes and not offer to merge the uploads