[Frontend] Add OpenAI API support for input_audio

[kylehh]

Added support for OAI's input audio API

# Use type 'input_audio` inside the message
messages=[
        {
            "role": "user",
            "content": [
                { 
                    "type": "text",
                    "text": "What is in this recording?"
                },
                {
                    "type": "input_audio",
                    "input_audio": {
                        "data": encoded_string,
                        "format": "wav"
                    }
                }
            ]
        },
    ]

1. The format is prefixed to the byte64 string following current vllm format
   f"data:audio/{input_audio_format};base64,{input_audio_data}"
2. There is no output audio support yet, so no support for modalities and audio

# Not supported
modalities=["text", "audio"],
audio={"voice": "alloy", "format": "wav"},

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[DarkLight1337]

Can you update this page and add some examples here as well? Thanks!

[DarkLight1337]

Thanks for adding this! I have polished the docs a bit before merging.