ParserInterface: add the BLANK_NODES_PRESERVE constant

There are some corner cases (e.g. parsing reference test outputs for the RDF-XML parser) where it is required to preserve blank node identifiers from the input data source. In such a case the rdfInterface\ParserInterface::BLANK_NODES_PRESERVE should be passed as the $baseUri parameter to the rdfInterface\ParserInterface::parse()/rdfInterface\ParserInterface::parseStream().