ensure sparql-engine-agnosticity

we curently make the [assumption](https://github.com/sdsc-ordes/kg-llm-interface/blob/e6c08fe4a34e3c846ec3815e544b811d0a31b818/aikg/flows/insert_triples.py#L48) that the update endpoint is at `<sparql-endpoint>/statments`, which is a GraphDB-specific thing. We should update `SparqlConfig` structure to ensure it is triple-store agnostic.