Support JSON response: CreateChatCompletionRequest class supports response_format field

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Supports specifying the response return format, either JSON or text. Address #105

See
OpenAI API document of response_format parameter
GPT-4-turbo and GPT-3.5-turbo-1106 support this parameter.

Specifying the return result as JSON is useful for workflows.

Code sample:

 _task = _openAi.CreateChatCompletion(new CreateChatCompletionRequest() { Model = "gpt-3.5-turbo", Messages = new List<ChatMessage>() { new() {Role = "system", Content = "Output JSON"}, new() {Role = "user", Content = "What's US and China's population?"} }, ResponseFormat = ResponseFormat.JsonObject }); 

Response:

{ "country_populations": { "United States": "331.9 million", "China": "1.41 billion" } } 

If you removed the ResponseFormat = ResponseFormat.JsonObject, the response becomes:

As of September 2021, the population of the United States is approximately 331 million and the population of China is approximately 1.4 billion. 

Note: According to the document, when using JSON mode, you must also instruct the model to produce JSON yourself via a system or user message.