Support export rdf graph in CSV format

Hey,

I noticed that currently export-rdf only supports exporting files in format of turtle, nquads, ntriples, neptuneStreamsJson and neptuneStreamsSimpleJson. Is it possible to support exporting files in CSV format? So that we can using SELECT tuple queries in SPARQL and export results in CSV format to be used by data warehouse.
DW usually doesn't have a good support for files in RDF format.

Seems it already supports tuple queries but io doesn't support CSV writer.

Thanks,
Weiran

Start by tracing the export-rdf handling for SELECT tuple queries and the IO writer implementations, since the issue identifies missing CSV writer support but no files or tests. The work is done when tuple-query results can be exported as CSV for downstream data-warehouse use, with coverage for the new format.

Written by the indexing model from the issue text.