Add Whisper feature extraction methods

[iyaja]

This PR integrates adds methods to obtain Whisper input features, embeddings, and transcripts from an AudioSignal.

• Introduces the WhisperMixin class with methods to set up the Whisper model, obtain input features, generate transcripts, and extract embeddings.
• The setup_whisper() method initializes the Whisper model and processor using the transformers library.
• The get_whisper_features() method resamples the input signal to the required sampling rate, processes the raw speech, and returns the input features.
• The get_whisper_embeddings() method extracts the embeddings from the input features using the Whisper encoder.
• The get_whisper_transcript() method generates the transcript from the input features and decodes it into text.

This new functionality allows developers to leverage the Whisper model for a wide range of audio processing tasks within the audiotools library.

[sotelo]

@pseeth could you also review this PR? There's things for which I lack context, for instance if we should implement this as a Mixin class?

[sotelo]

Thank you @pseeth !