feat: Add Google Gemini and OpenRouter providers

[fhalamzie]

This PR adds support for Google Gemini and OpenRouter as additional LLM providers for the claude-code-proxy.

🚀 Changes

New Providers Added

• Google Gemini - Full integration with Google's Gemini API
• OpenRouter - Access to 100+ models through a single API

Implementation Details

• ✅ Added gemini.go provider with Anthropic-to-Gemini request/response conversion
• ✅ Added openrouter.go provider supporting all OpenRouter models
• ✅ Updated config structure to support new providers
• ✅ Added environment variable support:
  • GEMINI_API_KEY, GEMINI_BASE_URL
  • OPENROUTER_API_KEY, OPENROUTER_BASE_URL
• ✅ Updated config.yaml.example with new provider configurations
• ✅ Full support for both streaming and regular responses

💡 Benefits

• Enables routing Claude Code subagents to Gemini models (gemini-1.5-pro, gemini-1.5-flash)
• Access to all OpenRouter-supported models (GPT-4, Claude, Llama, Mistral, etc.)
• More flexibility in model selection for different tasks
• Cost optimization by routing to different providers based on task requirements

📝 Example Configuration

```yaml
providers:
gemini:
api_key: "YOUR_GEMINI_KEY"

openrouter:
api_key: "YOUR_OPENROUTER_KEY"

subagents:
enable: true
mappings:
code-reviewer: "gpt-4o"
data-analyst: "gemini-1.5-pro"
doc-writer: "openrouter/claude-3-haiku"
```

✅ Testing

• Compiled and tested locally
• No API keys included in the code
• All provider implementations follow existing patterns

📚 Documentation

Updated config.yaml.example with comprehensive examples and environment variable documentation.