~aleteoryx/lfm_embed

ref: ca7c46e535ac60a976a8c60477fa9424c46b6e3f lfm_embed/src/cache d---------
whitelist -> allowlist
Add SPDX info to all files
Move caching to src/cache/; Finalize API parsing fixes

Font and user cache code has been moved to special files, independant from src/config.rs

API parsing changes have been properly tested, and last.fm API JSON is now trace-logged for debugging convenience.