- Add `FLAC_FLAC` (16), `FLAC_FLAC_24BIT` (22) format codes (based on librespot-org/librespot#796, librespot-org/librespot#1424)
- Regenerate Metadata_pb2.py with protoc 3.20.1
- Add enums: `SuperAudioFormat.FLAC`, `AudioQuality.LOSSLESS`
- Refactor to generic DRY `FormatOnlyAudioQuality` base class while maintaining existing `VorbisOnlyAudioQuality` as wrapper
- Remove `AAC_24_NORM` (replaced by `FLAC_FLAC` at code 16)