Enable RDF data formats in `%load` for Neptune Analytics

[michaelnchin]

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Neptune Analytics recently added support for RDF graphs: https://aws.amazon.com/blogs/database/build-and-deploy-knowledge-graphs-faster-with-rdf-and-opencypher/

In addition to the existing PropertyGraph data formats, Neptune Analytics graphs can now also ingest RDF data in n-triples format. We should allow-list n-triples as an option when submitting batch load requests via the %load magic.