Support Streaming of Function Call Arguments

[devtalker]

Summary

This PR implements real-time streaming of function call arguments as requested in #834. Previously, function call arguments were only emitted after the entire function call was complete, causing poor user experience for large parameter generation.

Changes

• Enhanced ChatCmplStreamHandler: Added real-time streaming of function call arguments during generation
• New streaming logic: Function call arguments now stream incrementally as they are generated, similar to text content
• Backward compatibility: Maintains existing behavior for completed function calls
• Comprehensive testing: Added tests for both OpenAI and LiteLLM models
• Example implementation: Created demonstration code showing the new streaming capability

Closes #834

[devtalker]

Hey @seratch @rm-openai! 👋

Would you mind taking a look at this PR? I've implemented real-time streaming for function call arguments (fixes #834). Basically, instead of waiting for the entire function call to complete, users now see the arguments being built up incrementally as the LLM generates them. This should make the experience much smoother for things like code generation and API building.

Let me know what you think! 🚀