Add LangChain Support to RAG Agent

[cmgzn]

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name: Pull Request about: Create a pull request

Description

1. Integration of LangChain Knowledge: Added the necessary components to support LangChain knowledge within the RAG agent.

2. Dynamic Backend Selection: Add the parameter "backend_engine" to knowledge_bank, allowing for manual input of the knowledge base backend (llama_index or langchain), or automatic selection based on the knowledge-config imported module.

3. Add LangChainAgent

Checklist

Please check the following items before code is ready to be reviewed.

• [x] Code has passed all tests
• [x] Docstrings have been added/updated in Google Style
• [ ] Documentation has been updated
• [x] Code is ready for review

[DavdGao]

Thanks for your contribution, we will review this PR as soon as possible.

[DavdGao]

@cmgzn Please resolve the conflict. @ZiTao-Li @FredericW @xieyxclack Please check this PR

[DavdGao]

Closing for new version preparation