Add stream_options property to get token usage.

[dominic-deantonio]

Added the ability to receive token usage based on this object in the OpenAI documentation:

https://platform.openai.com/docs/api-reference/chat/streaming

Added an optional parameter called streamOptions to the createStream interface which passes the aforementioned body parameter to the chat endpoint which enables receiving the optional usage statistics.

Added tests as well which succeeded during my tests. Some of the new tests might be considered unnecessary and I will be happy to change them if necessary.

Unrelated to the changes I made - I will note the following expects failed so I commented them out during my testing - they don't seem related my code changes:

    expect(streamEvent.choices.first.delta.content,
          isA<List<OpenAIChatCompletionChoiceMessageContentItemModel>?>());
      expect(streamEvent.choices.first.delta.content?.first?.text,
          isA<String?>());

Kept getting bad state: no element - presumably because we are using the .first getter here.

Thanks!

This PR is a solution to this issue: #155 (comment)

[imhuwq]

Good job, look for this feature too, and I think this PR is what I want.
When will this be merged and released?

[dominic-deantonio]

Yeah no response from the maintainer so far so not sure what to do. I'm just using my own fork for now