feat: add Output field to FunctionCall struct

[harnyk]

Describe the change

• Added the Output field to the FunctionCall struct as described in the OpenAI API docs.
• This field is optional and is of type *string.

Provide OpenAI documentation link https://platform.openai.com/docs/api-reference/run-steps/step-object#run-steps/step-object-step_details See: step_details / Tool Calls / tool_calls / Function tool call / function / output

Describe your solution The output field is provided with FunctionCall in the Assistants API context, when the function result has been already submitted.

Tests Unit tests updated, see the changes.

Additional context I developed a CLI tool for retrieving and viewing OpenAI assistant thread history using Assistants API: https://github.com/harnyk/threadviewer/

But I had to vendor and patch go-openai library in-place in order to make it support function call output field:

• The patch: https://github.com/harnyk/threadviewer/blob/master/openai-client.patch
• https://github.com/harnyk/threadviewer/blob/master/apply-patch.sh