Add ToolChoiceType::Required

[hanselke]

Adds required feature of toolchoice

[dongri]

https://platform.openai.com/docs/api-reference/chat/create#chat-create-tool_choice

[dongri]

Controls which (if any) tool is called by the model. none means the model will not call any tool and instead generates a message. auto means the model can pick between generating a message or calling one or more tools. required means the model must call one or more tools. Specifying a particular tool via {"type": "function", "function": {"name": "my_function"}} forces the model to call that tool.

[dongri]