Add support for assistants streaming

[weaseldotro]

The assistants API now supports streaming responses:

• https://platform.openai.com/docs/assistants/overview/step-4-create-a-run?context=with-streaming
• https://platform.openai.com/docs/api-reference/assistants-streaming

[weaseldotro]

I've forked this repo and quickly hacked a CreateRunStream function: https://github.com/weaseldotro/go-openai/commit/9ffb38f63bf2c936f95db9bce41efb7b392efbf1

This works for me just fine, but somebody should do a proper job with this, like add tests and docs and stuff.

[tanzyy96]

I also hacked something together on my own fork.

I had to add on streaming for some other endpoints like submitting tool outputs and just streaming from an existing thread. (if interested, can take a look at SubmitToolsOutputStream, CreateRunStreaming and CreateThreadAndStream. I know, I know, the naming's kinda shite.