Add CreateChatCompletionStreamResponse as an Event-stream Response Type to the /Chat/completions Endpoint

[paulhdk]

In the /chat/completions endpoint definition under x-oaiMeta it says:

x-oaiMeta:
    name: Create chat completion
    group: chat
    returns: |
        Returns a [chat completion](/docs/api-reference/chat/object) object, or a streamed sequence of [chat completion chunk](/docs/api-reference/chat/streaming) objects if the request is streamed.

The "streamed sequence of chat completion chunk objects" refers, I believe, to the CreateChatCompletionStreamResponse type, which is defined, but never explicitly linked to the createChatCompletionRequest() function.

Since it is never defined that createChatCompletionRequest() may return a stream of chunk objects, tools such as the swift-openapi-generator package, which I have been using to implement the OpenAI OpenAPI spec, cannot generate the correct code for the spec.
For more context, see the corresponding issue in the swift-openapi-generator repo.

This PR links the CreateChatCompletionStreamResponse type to the createChatCompletion() function.

[PSchmiedmayer]

Thank you @paulhdk for creating the PR; it would be amazing to see this merged to allow generators to pick up on this information and correctly parse steaming requests.