feat: Add LibriSpeech dataset builder(audio dataset)

[Anurag9Dhiman]

• Added Arrow-backed BaseDatasetBuilder implementation for LibriSpeech
• Supports train-clean-100 and test-clean splits from openslr
• Parses both audio (.flac via soundfile) and transcriptions
• Added comprehensive unit tests and exposed via timeseries module

What does this PR do?

Fixes # (issue)

This PR adds support for downloading and extracting the LibriSpeech dataset natively into stable-datasets using the optimized BaseDatasetBuilder architecture.

Changes Made:

Added LibriSpeech class in stable_datasets/timeseries/librispeech.py implementing the official base interface.
Audio Decoding: Efficiently parses .flac waveform subsets into float32 arrays using the soundfile library (a lighter, more efficient alternative to full PyTorch audio dependencies).
Transcriptions: Safely handles mapping text transcriptions from internal .trans.txt files inside the multi-directory .tar.gz archive.

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