[Graph RAG] Init Commit with GraphRag interfaces

[randombet]

Why are these changes needed?

Graph RAG is a long-sought feature from the community, we plan to support this and integrate FalkorDB and MS GraphRAG.
Please refer to the design doc for more details.
https://docs.google.com/document/d/15uuNrJrxujX8biPCAZzAtmeMKOFB4SEMkVszcE9nzRo/edit

This is the first PR to set up the graph rag interfaces.

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/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.

[randombet]

@sonichi I updated the interface, change graph store to graph query engine. This will make the graph rag agent interface clear.
Also, for some reason I am still not clear, graph rag seems to be closely correlated to the graph db.
Llama index has a more generic graph rag module (retriever, node processor, synthesizer/generation).
FalkorDB and MS GraphRAG have its own module.

Using graph query engine also helps us to leverage those above modules.

[randombet]

As per our discussion, add graph rag through capability and add a simple unit test.
@sonichi

[sonichi]

LGTM