Provide a "raw" sparql query operation

[tpluscode]

that would mean to use a generic http fetch instead of the barnard59-sparql operations? Because my understanding is that the 'construct' operation returns a stream of quads

Originally posted by @giacomociti in #202 (comment)

A hypothetical operation would use RawQuery from the sparql-http-client which does not parse. Maybe

@prefix sparql-raw: <https://barnard59.zazuko.com/operations/sparql/raw/> .

sparql-raw:construct ...

[giacomociti]

I added a raw/construct but in the cube fetch pipeline there is no perf gain at all (sometimes it's even slower without the parsing/serialize roundtrip)