Add support for openAI LLM

[Anash3]

[umiyuki]

OpenAI now supports Structured Outputs mode in its API. This may be very compatible with llama-cpp-agent; it would be helpful to have such an option to allow people without GPUs to use the application. https://openai.com/index/introducing-structured-outputs-in-the-api/

[Maximilian-Winter]

@umiyuki I'm working on a unified framework https://github.com/Maximilian-Winter/ToolAgents

[umiyuki]

I was not aware of the ToolAgents project. Great project! Looks very good.