Feature/add image generate model

[Jakevin]

把 #299 要求的功能加上

OPENAI_IMAGE_GENERATION_MODEL //使用的產圖模型，預設 dall-e-2
OPENAI_IMAGE_GENERATION_QUALITY //使用的產圖尺吋，預設 256*256

修正 當使用 dall-e-3時，但尺吋為256x256時，自動改為 1024*1024

實測：

[vercel]

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