feat: add getDataframeWithMetadata for RDFReader

[AuPath]

Closes #62. Add support for returning SPARQL SELECT binding metadata according to the SPARQL JSON Results format. Each binding now includes its value, RDF term type (URI, blank node, or literal), datatype (e.g., xsd:string), and language tag when applicable.

This functionality is exposed through a new method rather than getDataframe(), preserving backward compatibility while making the feature opt-in.

An example usage in an MTL mapping can be seen here:

mapping-template/src/test/resources/metadata/template.vm

Lines 7 to 10 in 9c0a0a7

	#set ( $rows = $reader.getDataframeWithMetadata("SELECT ?s ?name ?label WHERE { ?s a gtfs:Agency ; foaf:name ?name ; rdfs:label ?label . }") )
	@prefix ex: <http://example.com/> .
	#foreach($row in $rows)<$row.s.value> ex:name "$row.name.value" ; ex:nameType "$row.name.type" ; ex:nameDatatype "$row.name.datatype" ; ex:label "$row.label.value" ; ex:labelLang "$row.label.lang" .
	#end

The map containing the value and metadata is accessed by using the binding specified in the sparql query. Then the 'value', 'type' and other metadata can be accessed using dot notation via standard keys which are fixed ('value', 'type', ...).

[marioscrock]

Can this be encoded with some sort of "standardised value"? I would propose using the canonical IRIs already as values in the returned dataframe

rdf:IRI
rdf:BlankNode
rdfs:Literal

(note: Literal is under rdfs, the other two under rdf namespace)

[marioscrock]

Use xsd:language data type for labelLang— http://www.w3.org/2001/XMLSchema#language

ex:labelLang "it"^^xsd:language

I know it's just a test, but we want to provide good example 😄