Add option to keep query prefixes

[vitalis-wiens]

The current implementation supports to generate either all registered prefixes or only used prefixes from the query body. However, several database vendors provide means to add optimization hints for the execution of the sparql query via Prefix definition.

For Example when configuring the Parser in the following way

var parser = new SparqlParser({
      prefixes: {
        schema: "https://schema.org/",
        owl: "http://www.w3.org/2002/07/owl#"
      }
    });

and having the following query

PREFIX db_opt: <https://db.control.param_10> 
SELECT * WHERE {
  ?s a owl:Class
}

The available options will either print all three prefix definitions (schema, owl, db_opt) or only owl.

We would like to keep the Prefix that is in the Prefix definition of the query and use only the used Prefixes of the query body.

This should result in a query for database vendor should look as follows:

PREFIX db_opt: <https://db.control.param_10> 
PREFIX owl: <http://www.w3.org/2002/07/owl#>
?SELECT * WHERE {
  ?s a owl:Class
}