Add Google Gemini backend implementation

- Added APIType_GoogleGemini constant to uctypes
- Created gemini package with backend, types, and message conversion
- Implemented streaming support with SSE
- Added tool calling (function calling) support
- Added support for images, PDFs, and text files
- Integrated gemini backend into usechat-backend.go

Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>

Author: Copilot