[OpenAI] Introduce a new option class for the streaming methods, getCompletionsStream and getChatCompletionsStream

[mssfang]

Rethink and redesign the CompletionsOption and ChatCompletionsOption classes.

Since we have separate method for both of streaming and non-streaming completions and chat completions methods, we should separate the optional classes into ChatCompletionsOption and a new ChatCompletiondsStreamOption, as well as CompletionsOption and a new CompletionsStreamOption.

For example,

• ChatCompletionsStreamOption should have stream=true by default and include all parameters related only to streaming.
• ChatCompletionsOption should not include parameters related only to streaming.