Fix/structured output

[Aktsvigun]

This PR incorporates the response format in the messages when using the structured outputs. It works as follows:

1. If the json schema itself, or it "prettified" version with json.dumps is already present in the messages, it won't be added again.
2. Otherwise, if the system message exists, the response format will be added at the end of this message as following:
   system_message = system_message + f"\n\nGenerate the response in the following JSON format:\n{format}"
3. Otherwise, the response format will be added to the last message.

[github-actions]

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