It can be very annoying if we want to use something like boolval, but have to define an entire anonymous function to pass it in. This PR adds support for tpassing it in directly as a string, like is posible with User::registerPreference.
Because invokable class objects are not directly called and instead it's the callback wrapper that calls these objects, it's currently not possible to receive arguments by reference on an invokable class.
To fix this we pass the arguments by reference by default when calling the object in the callback wrapper.
- Standardize signatures and variable names for extenders that take callbacks
- Adjust model extender docblock to clarify that default calue can't be an invokable class.
- Make invokable classes provided to Model->relationship
- Add integration tests to ensure Model->relationship and User->groupProcessor extenders accept callbacks
- Extract code for wrapping callbacks into central util
