allow custom API URLs in ChatGPTSession

[Vokturz]

This change allows a custom api_url to be used when creating a ChatGPTSession.

Previously, if you instantiated AIChat with a custom api_url as a string, it resulted in an error when httpx attempted to send the request. For instance:

ai = AIChat(api_key='None', api_url='http://localhost:8000/v1/chat/completions', console=False)

Invalid type for url. Expected str or httpx.URL, got <class 'pydantic_core._pydantic_core.Url'>: Url('http://localhost:8000/v1/chat/completions')

Now, the api_url is cast to a string before it is used with httpx. This effectively removes the error and enables the use of a custom API URL.

This can be useful in scenarios where requests need to be rerouted to a local server or a different API endpoint for development or testing purposes. A practical example of this is when using an open-source Language Model with an OpenAI-like API web server, such as the one provided by llama.cpp.