[WIP] Review aisuite for implementing function calling

[Copilot]

• Understand the existing codebase and how function calling works in aisuite
• Modify Providers/xAI.ps1 to support function definitions in API calls
• Enhance the response handling in Providers/xAI.ps1 to check for function calls
• Update the return structure in Providers/xAI.ps1 to include function call information
• Modify Invoke-ChatCompletion.ps1 to handle function calling responses
• Add function execution logic to Invoke-ChatCompletion.ps1
• Add ability to pass results back to the model for continued conversation
• Add documentation and examples
• Test implementation

The implementation adds support for function calling with the xAI provider in PSAISuite. The changes follow these key aspects:

1. Added Functions parameter to Invoke-XAIProvider to support passing function definitions to the API
2. Updated response handling in Invoke-XAIProvider to detect function calls and return the appropriate data
3. Added Functions parameter to Invoke-ChatCompletion and enhanced it to:
   • Pass function definitions to provider implementations
   • Handle function call responses from the model
   • Execute PowerShell functions when the model requests them
   • Pass function results back to the model
4. Updated tests to verify the new functionality
5. Added a comprehensive example demonstrating how to use function calling with xAI

Fixes #27.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.