Neo4j Graph Rag Integration

[Eric-Shang]

Why are these changes needed?

This PR integrates Neo4j Graph Rag. It wraps Neo4j Property Graph Index as a GraphQueryEngine and could be added to an AG2 agent through GraphRagCapability.

I also include a notebook example to show how to use this feature.

Related issue number

part of #68

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[Eric-Shang]

Need to add functionality to allow pre-defined schema and an unit test. The notebook functions as an integration test.

[AgentGenie]

Please add unit tests

[AgentGenie]

Debugged a random pydantic exception
1 validation error for KGSchema triplets Field required [type=missing, input_value={}, input_type=dict] For further information visit https://errors.pydantic.dev/2.9/v/missing'

Resolved by downgrade the packages to
%pip install llama-index==0.11.15 llama-index-graph-stores-neo4j==0.3.0

[AgentGenie]

tentative approval for testing