feature required: `add_node`, `add_edge` instead of `add_document`

[kwang2049]

We have no methods for adding nodes/edges. The current code relies on add_document

square-core/datastore-api/app/routers/kgs.py

Line 270 in 2ffed37

successes, errors = await conn.add_document_batch(kg_name, documents)

It could be better to add new methods in the kg_connector

[timbmg]

@kwang2049 what is the status of this?