Make SPARQLModelAdapter.query asynchronous

[lu-pl]

SPARQLModelAdapter.query should be asynchronous.

Especially SPARQLModelAdapter._query_paginate_grouped and SPARQLModelAdapter._query_paginate_ungrouped should support async; currently those methods execute the main query and the counter query synchronously.

[lu-pl]

Update: Since SPARQLModelAdapter uses strategies for SPARQL query execution now (defaulting to HttpxStrategy), making SPARQLModelAdapter query execution asynchronous would either mean specialcasing the SPARQLWrapperStrategy which cannot be made async, or dropping support for SPARQLWrapper entirely and moving to httpx for SPARQL query execution.

Clearly, moving to httpx entirely is the better option.

[lu-pl]

httpx uses an httpx.AsyncClient context for asynchronous calls, its interface is widely identical to the httpx.post callable used for running SPARQL queries, so moving to async query execution should not be too complicated.