Add service for publishing APKs in F-Droid repositories

The first commits are mostly refactorings to prepare for the new service. The last commit adds the new service. fdroidpublisher replaces the publishing of APKs in our F-Droid repositories currently done by Binary Factory, i.e. it replaces the "F-Droid Publishing" stage of the Android Jenkins pipelines (https://invent.kde.org/sysadmin/binary-factory-tooling/-/blob/master/craft/pipeline-templates/android.pipeline) and, in particular, the generaterepo.py script (https://invent.kde.org/sysadmin/binary-factory-tooling/-/blob/master/android/generaterepo.py).

I have tested this with Itinerary: https://invent.kde.org/pim/itinerary/-/pipelines/489762

@bcooksley @vkrause @apol Please have a look.

Currently, as on Binary Factory, a single APK is published. I think in a second step it makes sense to extend the service, so that the APKs for all architectures of an app can be published together.