~aleteoryx/lfm_embed

ref: 27bd2e5f5699fd9101aa298993c3a9045caf02f4 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.