Introduction of `rml:Sparql` as a reference formulation

[chrdebru]

I wanted to open an issue to prepare for the SPARQL Reference Formulation to be introduced in RML-IO. The different SPARQL SELECT return formats should be removed and rml:SPARQL as a reference formulation should be introduced.

[DylanVanAssche]

We can discuss this, but rather in the IO Registry repo as SPARQL will not be part of the WG charter