Determine HTTP query arguments for TPF requests automatically

[hartig]

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

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