Adding graph_aware for `rdflib.Dataset`

[bollwyvl]

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Thanks for this library!

I'd love to use rdflib-sqlalchemy with rdflib.Dataset, and have started tinkering with an subclass approach for my use case.

Are there any pointers for implementing stores with graph_aware = True? It looks like add_graph and remove_graph need to be added, but I'm imagining it would also require a new table and reworking some query gymnastics to handle empty graphs.

Would this be something of interest as a PR, or is it beyond the scope of this repo?