Constrain `response_format` to documented accepted values

[nikeee]

Essentially, it takes the same options as transcribe, as done in #58.

Ref in docs: https://platform.openai.com/docs/api-reference/audio/createTranslation#audio-createtranslation-response_format

Ref in transcriptions: https://github.com/openai/openai-openapi/blob/23a4b18b65bb7b002894bae1084d99134b2897e3/openapi.yaml#L7271-L7281