feat: add FLAC lossless format support

## Summary - Adds **FLAC lossless audio format support** to librespot-python based on [format codes in librespot upstream](https://github.com/librespot-org/librespot/blob/61f517bfff27c8cdb209cd36ffefc682285b540d/protocol/proto/metadata.proto#L309-L313) - Refactors quality picker classes for DRYness while maintaining backward compatibility ## Details - Add `FLAC_FLAC` (16) and `FLAC_FLAC_24BIT` (22) format codes to `metadata.proto` - Added upstream by librespot-org/librespot#796 and librespot-org/librespot#1424 - Regenerated `Metadata_pb2.py` with protoc 3.20.1 (as in requirements) - Removed obsolete `AAC_24_NORM` as done upstream (format that previously occupied code 16) - Add enum values for FLAC: - `SuperAudioFormat.FLAC` maps FLAC formats to FLAC container type - `AudioQuality.LOSSLESS` maps FLAC formats to lossless quality tier - Refactor to generic DRY `FormatOnlyAudioQuality` base class, while maintaining 100% backward compatibility via: - Maintaining existing `VorbisOnlyAudioQuality` as a wrapper - Adding new `LosslessOnlyAudioQuality` wrapper for FLAC file selection - Also fixes minor bug within unknown-format error in decoders.py