Allow users to pass an httpx.Client/httpx.AsyncClient instance for query execution

[lu-pl]

It should be possible for users to pass an httpx.Client/httpx.AsyncClient instance to RDFProxy's query execution utility. This allows for significant performance improvements, see Why use a Client?.

Currently, there are two strategies implemented for query execution, rdfproxy.SPARQLWrapperStrategy and rdfproxy.HttpxStrategy. The SPARQLWrapperStrategy will be discontinued shortly and the HttpxStrategy will probably be replaced with an httpx-based SPARQLWrapper class, which will implement an asynchronous query method as well as a synchronous wrapper method. See #49 .
So this issue mainly concerns this new SPARQLWrapper class.