Add ToolKit for new interface

[koxudaxi]

The PR adds ToolKit for a new style interface.

class BookRecommendationTools(Toolkit):
    """A toolkit for recommending books."""

    reading_level: Literal["beginner", "advanced"]

    @toolkit_tool() # not sure about this naming...
    def format_book(self, title: str, author: str) -> str:
        """Returns the title and author of a book nicely formatted.

        Reading level: {self.reading_level}
        """
        return f"{title} by {author}"

@openai_call(model="gpt-4o")
def recommend_book(genre: str, reading_level: Literal["beginner", "advanced"]):
    """Recommend a {genre} book."""
    toolkit = BookRecommendationTools(reading_level=reading_level)
    return {"tools": [toolkit.create_tools()]}

The broad policy was discussed within #278, but any undecideds will be resolved within the PR.
Closes #278

[koxudaxi]

@willbakst
I have made a rough implementation based on what we have discussed. I have not implemented create_tool or unittest yet, but first, I want to ensure that my implementation policy is not different from your assumptions.
Also, I have not confirmed my implementation policy for the part of the connection with the new interface (@openai_call), which is not yet completed, so it is currently a TODO.
Of course, type hints and variable names may need improvement, but please confirm the general framework.