Stream client for chat completion, entities update and some junit

[Simon-Wong-hjz]

Hi @reactiveclown,

Hope this finds you well. I'm working on my GPT project and found this repo. I need a stream client to support my use cases but I see that the client for streaming is still pending implementation according to #2. So I add an SSE client for chat completion here and hope this will initiate something :)

I’m available to discuss any aspects of the PR if needed. Please feel free to reach out if you have any questions.

Best Regards, Simon Wong