cached scanner signatures

This adds caching capabilities to fdroid scanner. It also introduces a new json based data format for storing scanner signatures. The goal is to have an intermediate format fdroid scanner understands and convert signatures from various sources (eg. exodus, mobil sicher, etc.) into this format to help packagers identify inclusion policy issues and evidence for anti-features more quickly.

Here's a preview of an additional project for collaboratively editing these signatures the same way fdroiddata works: https://gitlab.com/uniqx/fdroid-scanner-signatures I've transcribed definitions from scanner.py into the new collaborative yaml-format there. It also uses gitlab pages to assemble json files for fdroidserver scanner: https://uniqx.gitlab.io/fdroid-scanner-signatures/

This feature also comes with an additional project for collaboratively editing signatures the way fdroiddata works: https://gitlab.com/fdroid/fdroid-suss

closes #1008 (closed)