Switch cookie-filter API to std::function

Allows us to use return value instead of magic properties

Task-number: QTBUG-66825
Change-Id: I55202a7aca2e662bc214b05caf767d2da496f026
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>