Determine HTTP query arguments for brTPF requests automatically

[hartig]

The four arguments for providing the subject, the predicate, the object, and the solution mappings of brTPF requests are typically "subject", "predicate", "object", and "values". However, this may not be the case for all brTPF servers. Instead, brTPF servers can chose what these arguments are and declare them in the Hydra description returned in each response to a brTPF request.

Currently, HeFQUIN can use only the aforementioned typical argument names, which are hard-coded in BRTPFInterfaceUtils. We have to change that and extract the argument names from the Hydra description.

This issue is the brTPF variant of issue #232 for TPF.