* feat: add graph_stores, impl Simple KG & Nebula KG

* fix logan-markewich's comments

Still need a followup commit to address backwards compatibility
of graph_store.json from the previous impl.

* chore: fix lint and ut regression

* SimpleGrpahStore from_persist_path lagacy kg json file

* improve legacy support

* linting

* modify attribute name

* typing

* address simon comments

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Co-authored-by: Logan Markewich <logan.markewich@live.com>