Add LlamaIndex callback handler

[areibman]

🚀 Feature Request

💡 Got a brilliant idea? Similar to Langchain, Llama Index also provides callback handlers. This is a potential opportunity to give built-in observability to agents built with this framework.

🤔 Is your feature request related to a problem? A user in TokenCost might be a good initial pilot.

🎉 Describe the solution you'd like Implement the callback handler here: https://docs.llamaindex.ai/en/stable/api_reference/callbacks.html

🔍 Additional context Before working on this, find at least a handful of LlamaIndex agent builders and trial with them