Add LLM: Google Gemini Pro

please add Google Gemini to the llm, Thank you!

Look at existing LLM implementations in the codebase, likely under a directory like `src/llm` or `src/chains`. Examine how providers like OpenAI are integrated, including API client setup, request/response handling, and error management. Check for any existing Gemini-related issues or PRs. The work involves implementing a new struct for Gemini Pro, conforming to the existing LLM trait, and adding tests. Start by reading the project's contributing guidelines and understanding the architecture.

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