Draft: Add config files for APK signing and Flatpak signing/publishing

Ingo Klöcker requested to merge work/kloecker/signing-configs into master

These are preliminary settings used for test-running the services.

@thiagosueto I have added some documentation about configuring the soon-to-be-put-into-production KDE CI Notary Services here. This documentation is (supposed to be) referenced from the pipeline documentation I have added in !144 (merged). Again, comments on the documentation are welcome.

Edited by Ingo Klöcker

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