Add support for models via LiteLLM proxy server

**Is your feature request related to a problem? Please describe.**
I’d like to add support for OpenAI models using the [LiteLLM simple proxy](https://docs.litellm.ai/docs/simple_proxy). This will expand the model compatibility of adk-java to include OpenAI endpoints via a standardized proxy interface.

**Describe the solution you'd like**
-The plan is to use Java's HTTP client to call the proxy endpoint and integrate the response into the existing ADK pipeline.