Add Google Gemini support

[lucasliet]

• add prompt to put google gemini api key on installation
• add condition to use gemini api key on litellm if model name starts with gemini/*
• change use litellm default value to true
• change default model to gemini/gemini-1.5-flash

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since gemini has free tier usage and litellm has native support to it, it is interesting to use it instead of openai gpt

[abhishekbhakat]

This PR #604 can also include Gemini. No need to create handling.