Add script to verify BitBanana github releases

This adds a script to verify BitBanana github releases. BitBanana was forced to do the release on google play store with AAB (Android App Bundle). On Github and F-Droid the app is still delivered as an APK. This script helps to verify those two cases.

For the PlayStore release, we will probably have to add a whole new procedure on how to verify such builds. I am open to help writing the necessary scripts for that as well.