Do you need to file a feature request?

• I have searched the existing feature request and this feature request is not already filed.
• I believe this is a legitimate feature request, not just a question or bug.

Feature Request Description

In the MongoDB implementation, we cache the MongoDB Client instance https://github.com/HKUDS/LightRAG/blob/main/lightrag/kg/mongo_impl.py#L49)

Therefore, the default behavior is to always route to one MongoDB database. It would be great if we can support multiple MongoDB databases to enable querying of different knowledge bases!

Additional Context

No response