mtmd: refactor audio preprocessing

[ngxson]

The goal of this PR is to allow more audio pre-processing mechanism to be added into mtmd

While the code is not very clean, this should already allow:

• Simplify model : add ASR support for LFM2-Audio-1.5B #17694
• Potentially support gemma 3n audio

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Key points

• Each model's preprocessor now have their own subclass extended from mtmd_audio_preprocessor
• Preprocessor can access hparams directly (to read audio params like n_mel, n_fft, etc)
• Each preprocessor also have its own initialize() function which will be called on model load, to initialize global cache entries like sin/cos, hann window
• Filter bank is now constructed dynamically thanks to @tdakhran 's implementation of fill_mel_filterbank_matrix (the hard-coded value is now removed)

[ngxson]

Hmm, I think I can also upstream some changes from #17694 , that would make your PR a bit shorter @tdakhran

I will remove the pre-calculated filters and replace with your version

Edit: since my goal is to implement conformer, I think I will end up copying a lot of code and refactor them along the way

[ngxson]

@ggerganov This is ready for review. I only have basic knowledge about signal/audio processing, would appreciate if you can have a deeper look to see if things are still correct compared to the original code from whisper.cpp

Note: this change also contain enough code for LFM2-audio and gemma 3n audio preprocessor

Test results:

[audio] OK: ggml-org/ultravox-v0_5-llama-3_2-1b-GGUF:Q8_0 [audio] OK: ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M [audio] OK: ggml-org/Voxtral-Mini-3B-2507-GGUF:Q4_K_M