feat(rag): add MongoDB vector store in the RAG module.

[qbc2016]

AgentScope Version

[The version of AgentScope you are working on, e.g. import agentscope; print(agentscope.__version__)]

Description

As the title says.

Checklist

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

• [ ] Code has been formatted with pre-commit run --all-files command
• [ ] All tests are passing
• [ ] Docstrings are in Google style
• [ ] Related documentation has been updated (e.g. links, examples, etc.)
• [ ] Code is ready for review