Support for OpenRouter API

[EduardoAzaldegui]

First of all, thank you for this excellent library! I've been using it and it's one of the cleanest Java implementations for OpenAI.

I would like to suggest adding support for OpenRouter. Since OpenRouter uses a standardized API format very similar to OpenAI's, it would be a great addition to allow Java developers to access a wide range of LLMs (Claude, Llama, Gemini, etc.) through this library.

Are there any plans to include OpenRouter as a supported provider? I believe it would significantly increase the flexibility of the library for projects requiring multi-model support.

Looking forward to your thoughts!