Implement function calling capabilities #21
Description
Currently the library doesn't support OpenAI's function calling feature. This feature allows structured data extraction and tool usage with AI models.
Proposed Changes
- Add function calling support to the OpenAIService
- Create appropriate models/DTOs for function definitions
- Add examples of function calling usage
- Support different function calling modes (none, auto, tool)
Benefits
- Enables structured data extraction from AI responses
- Allows for tool usage with AI models
- Supports more advanced AI application patterns