Add configurable SPARQL endpoint support for QLever in PagesProcessor

In ba2acee we added support for QLever as alternative
SPARQL query engine in PropertyStatistics.

Here we allow to configure its use from the Property dashboard template:

• add sparql_endpoint parameter to Property dashboard configuration
• implement SparqlEngineBuilder to create appropriate engine based on the URL, defaulting to Wikidata Query Service.
• pass the built engine to PropertyStatistics
• add comprehensive unit tests for all the new functionality

Bug: T385749