Allow api_url to be configured that gets used in chatgpt_rs

[goodgamgee]

First of all: Great project!

The OpenAI API is also available from other vendors (like Microsoft Azure) or there are compatible AI implementations or you have to use an API gateway.

Long story short: Please let the user configure a custom API endpoint for GPT.

Thanks!

[Hyde46]

Great points!
Will update and parameterise it accordingly