[Frontend] Add /v1/audio/transcriptions OpenAI API endpoint

[NickLucche]

Follow up on the great work by @robertgshaw2-redhat and @varun-sundar-rabindranath here #12458.

This PR adds the /v1/audio/transcriptions OpenAI API endpoint.

Basic example (start server with vllm serve openai/whisper-large-v3):

from openai import OpenAI
from vllm.assets.audio import AudioAsset

mary_had_lamb = AudioAsset('mary_had_lamb').get_asset_path()

openai_api_base = "http://localhost:8000/v1"
client = OpenAI(
    api_key="EMPTY",
    base_url=openai_api_base,
)
with open(str(mary_had_lamb), "rb") as f:
    transcription = client.audio.transcriptions.create(
        file=f,
        model="openai/whisper-large-v3",
        language="en",
        response_format="text",
        temperature=0.0)
    print("transcription:", transcription)

I am also adding a correctness test based on computing the WER for a subset of a dataset found here https://huggingface.co/datasets/open-asr-leaderboard/datasets, comparing against the transformers baseline.

Mind that this is all currently a bit "fit" to Whisper, being the only model we support. In particular in the way special tokens (<|startoftranscript|>, <|transcribe|>).. are sent as input, as this would require a more general design. Same thing in the validating supported languages and audio duration limit.

Let me know if I overlooked something in this PR.

TODOs (next PRs):

• verbose_json response format
• Group api validation (audio length limit above all) by model so we can extend to other models.
• Openai does has no "streaming" support, perhaps we could add it as a custom addition (shouldn't be too different from what we have).
• Translation API (more of the same with a token)

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[NickLucche]

Had to rebase and force push to make some very old commits compliant with dco and pre-commit