[Frontend] Add max-completion-token option to transcription/translation endpoints

[NickLucche]

Straightforward PR to allow users to specify max tokens generated on a STT endpoint, using an additional non-OAI argument.

[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request adds a max_completion_tokens option to the transcription and translation endpoints. The implementation in vllm/entrypoints/openai/speech_to_text.py has a critical bug that will cause a TypeError when max_completion_tokens is not provided, and an AttributeError for translation requests. I've provided a suggestion to fix this. To fully support this for translations, max_completion_tokens should also be added to the TranslationRequest protocol. Additionally, a test case for the translation endpoint with this new option would be beneficial.

[mergify]

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