New API feature: disable parallel function calling via `parallel_tool_calls: false`

In Chat Completions and Assistants API, when you use tools param for function calling, the models are allowed to perform multiple function calls together (aka parallel function calling) by default.

You can now disable this behavior by setting parallel_tool_calls: false in both Chat Completions and Assistants API, ensuring the model will call at most one function for each request. The tool_calls field on Completions and Run objects in the response is still array type, just never with more than one elements.