Add example to count tokens for structured responses

[jsh9]

In https://github.com/openai/openai-cookbook/blob/main/examples/How_to_count_tokens_with_tiktoken.ipynb, there are examples of counting tokens for regular messages and tool calls, but there isn't an example for structured responses.

Is it possible to add an example for it?

Thanks!

[github-actions]

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