[df] Improve user interface in DistrRDF

[martamaja10]

The tests of the new features (PR https://github.com/root-project/root/pull/16309) are added here.

The idea for the tests is to use the same function for filtering - check_number_less_than_5(rdfentry_) , but the function is delivered via the headers, shared libraries (note: we need to decide how do we want to test this as .so is in the gitignore), files or direct C++ code declaration. The issue with adding several functions with code declarations is yet to be investigated.