LlamaIndex LangChain LLMs Integration

The `llama-index-llms-langchain` library provides a seamless integration layer, allowing users to leverage LangChain's extensive collection of Large Language Models (LLMs) within the LlamaIndex framework. It acts as a bridge, enabling LangChain LLM instances to conform to LlamaIndex's LLM interface. The current version is `0.8.0`, and it receives regular updates aligned with LlamaIndex core releases.