feat: Add support for image function tools

[vaydingul]

This PR adds support for image function tools to the OpenAI Agents Python SDK.

It is inspired by the #341 .

Current function_tool implementation only allows output to be strictly string, which creates a problem when we want to pass image input in the request data. This PR tackles this problem by both providing a standart function_call_output and additional image-related arguments back-to-back.

What's included

• Added ImageFunctionTool class and image_function_tool decorator
• Implemented necessary support in the run implementation, models, and item handling
• Added example showing usage of image function tools (examples/tools/image_function_tool.py)

Usage

Use the @image_function_tool decorator to create tools that work with images:

@image_function_tool
def image_to_base64(path: str) -> str:
    """
    This function takes a path to an image and returns a base64 encoded string of the image.
    It is used to convert the image to a base64 encoded string so that it can be sent to the LLM.
    """
    with open(path, "rb") as image_file:
        encoded_string = base64.b64encode(image_file.read()).decode("utf-8")
    return f"data:image/jpeg;base64,{encoded_string}"

The tool can then be used to allow agents to process and analyze images.

Supporting example script is located at examples/tools/image_function_tool.py