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 https://github.com/UKP-SQuARE/square-core/blob/2ffed37ad32c7912c63bc6601c36bd2307fb0a74/datastore-api/app/routers/kgs.py#L270 It could be better to add new methods in the kg_connector

[timbmg]

@kwang2049 what is the status of this?