please implement the function_call feature similar to openAI

function_call
Controls how the model responds to function calls. "none" means the model does not call a function, and responds to the end-user. "auto" means the model can pick between an end-user or calling a function. Specifying a particular function via {"name":\ "my_function"} forces the model to call that function. "none" is the default when no functions are present. "auto" is the default if functions are present.
[https://platform.openai.com/docs/api-reference/chat/create](https://platform.openai.com/docs/api-reference/chat/create)

Start by reviewing the repository's relevant REST API specification and the linked OpenAI Chat Create API reference. Determine which API definition should expose function_call and how its none, auto, and named-function behavior is represented. Done means the specification clearly supports the requested option and its defaults, with appropriate validation or examples.

Written by the indexing model from the issue text.