feat: create volc cloud api example code

[ZhangSetSail]

Description This PR adds a new example demonstrating how to use the LangChain Go library with Volc Cloud API. The example shows how to:

Configure the OpenAI client to use Volc Cloud API endpoints Set up proper authentication with API keys Display real-time reasoning process from the model Format the final response in a clean way The implementation is based on the existing DeepSeek example but adapted for Volc Cloud's API specifications. This example will be useful for users who want to integrate Volc Cloud's LLM services with LangChain Go.

Implementation Details Uses the existing OpenAI client with custom base URL and authentication Demonstrates streaming capabilities to show reasoning process in real-time Includes proper error handling and output formatting All code is properly documented with English comments Testing The example has been manually tested with a valid Volc Cloud API key and endpoint ID, confirming successful connection and response generation.