Add Support for OpenAI-Compatible Providers

Currently, the Go version of the Agent Development Kit (ADK) is tightly coupled with Gemini/Vertex AI. While the Model interface exists, there are no built-in providers for OpenAI or other popular LLM services. This creates a high barrier to entry for developers who want to maintain a multi-cloud strategy or use local models (like Llama 3 via Ollama) using a common SDK. I would like to see an official provider (driver) for OpenAI-compatible APIs within the google.golang.org/adk/model package.

The Python version of many Google AI tools often has broader provider support. Bringing this parity to the Go ADK would significantly increase its adoption in the backend community where Go is a primary language.