[feature] support image output via OpenAI DALL-E model

[hsluoyz]

We have the following existing code, but it may not work very well. Need to fix existing issues and make it work.

casibase/model/local.go

Lines 255 to 264 in a265905

	modelResult := &ModelResult{}
	if getOpenAiModelType(p.subType) == "Chat" {
	if p.subType == "dall-e-3" {
	reqUrl := openai.ImageRequest{
	Prompt: question,
	Model: openai.CreateImageModelDallE3,
	Size: openai.CreateImageSize1024x1024,
	ResponseFormat: openai.CreateImageResponseFormatURL,
	N: 1,
	}