Add knowledge graph support for MongoDB in LlamaIndex by implementing Property Graph Index functionality with MongoDB as the backing store.

Context

LlamaIndex recently introduced the Property Graph Index, which offers advantages over traditional knowledge graph representations. Currently, LlamaIndex has strong MongoDB integration for vector search, document storage, and key-value operations, but lacks native support for storing and querying property graphs in MongoDB. Link: 

https://www.llamaindex.ai/blog/introducing-the-property-graph-index-a-powerful-new-way-to-build-knowledge-graphs-with-llms

Definition of done

•  Implement MongoDBPropertyGraphStore class that extends the PropertyGraphStore abstraction

•  Support storing nodes with labels, properties, and relationships in MongoDB collections

•  Enable querying nodes by ID, properties, and relationship traversal

•  Implement basic graph operations (insert, update, delete nodes and relationships)

•  Include comprehensive unit tests and integration tests

•  Update documentation with MongoDB property graph examples