[Frontend][Core] Add plumbing to support audio language models

[petersalas]

This adds infrastructure to support multi-modal audio language models.

• MultiModalDataBuiltins defines audio as a tuple of a numpy.ndarray and the sample rate. (librosa is used to parse the audio.)
• DEFAULT_PLUGINS now includes an AudioPlugin
• The OpenAI API frontend tentatively supports "audio_url" content parts, albeit with the same restrictions that currently apply to "image_url" parts (e.g. only one multi-modal chunk per prompt).
• A number of methods/types that were only nominally vision-specific are renamed to be more generically multi-modal. For example, SupportsVision is now SupportsMultiModal.

This PR does not include any such audio models, but does include a test that exercises the OpenAI frontend and audio pipeline (see tests/entrypoints/openai/test_audio.py).

FIX #7335

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[github-actions]

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[DarkLight1337]

Thanks for implementing this! Overall it looks good, just some small things that need to be resolved.

[DarkLight1337]

Are you going to work on adding a new audio model after this PR?

[ywang96]

This is exciting! Thank you for kicking off the initiative to support audio LMMs!

[petersalas]

Are you going to work on adding a new audio model after this PR?

If you're open to it, I'd love to bring https://github.com/fixie-ai/ultravox into vLLM. We're currently serving it via an externally registered model + some monkeypatches to handle audio via "image_url" but we'd certainly rather it be directly supported!

[DarkLight1337]

Going to merge this so I can incorporate it into my #7126 tomorrow.

[ywang96]

Just one change needed - Thank you for making this PR! @petersalas