Add Gemini Pro & Gemini Pro Vision support

[bi1101]

1. Get Google AI Studio API Key: You'll need to obtain an API key from ai.google.dev.
2. Put API Keys in the file gemini-api-key.json
3. Make API call with gemini-pro or gemini-pro-vision as the model name. All format are like the OpenAI Docs

gemini-api-key.json format:

{
    "api_keys": ["key1", "key2", "key3", ...]
}