Extend the RDF-based config to specify type of bind join for SPARQL endpoints

[hartig]

We have three different physical operators for performing a (batched) bind join at a SPARQL endpoint: PhysicalOpBindJoinWithFILTER, PhysicalOpBindJoinWithUNION, and PhysicalOpBindJoinWithVALUES. Currently, the filter-based one is used as the default when converting a logical plan into a physical one and this decision is hardcoded in the corresponding convert functions of LogicalToPhysicalOpConverter.

This issue is about extending the RDF-based configuration of HeFQUIN such that the decision configured. I am not sure yet how this can best be done. A starting point might be to extend the constructor of LogicalToPhysicalPlanConverterImpl.

(Note to myself: This is relevant only when using a physical optimizer that converts the given logical plan into a physical plan, which not all the optimizers that we have do. For instance, for the optimizers that create the physical join subplans in a bottom-up fashion (e.g., greedy or via dynamic programming) this is irrelevant; they consider all the possible types of bind join operators.)