[QRestAM] Use qxp::function_ref instead of <Functor> in private API

The QRestAccessManager private code uses templates to take in an encapsulating function.

This task is about replacing these with qxp::function_ref. Broadly speaking, as a non-owning function ref, it has severel advantages. But in this particular class I think the main benefit will be readability (and in future possibly also type safety and performance, depending on how the usage evolves).