### What problem does this PR solve?

This PR introduces the `ragflow_chat` plugin for the ChatGPT-on-WeChat
project, extending its functionality by integrating Retrieval-Augmented
Generation (RAG) capabilities. It allows users to have more contextually
relevant conversations by retrieving information from external knowledge
sources (via the RAGFlow API) and incorporating it into their chat
interactions.

The primary goal of this PR is to enable seamless communication between
ChatGPT-on-WeChat and the RAGFlow server, improving response accuracy by
embedding knowledge-based data into conversational flows. This is
particularly useful for users who need more complex, data-driven
responses.

This PR adds a new plugin that enhances ChatGPT-on-WeChat with Ragflow
capabilities, allowing for a more enriched conversational experience
driven by external knowledge.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)