Is there an existing issue for this?

• I have searched the existing issues
• I have read the guide to filing a bug

Use case

The new onAttach and onDetach in ScrollController are currently not usable with PageController since its constructor doesn't include them, and extending classes can't set the fields on ScrollController

As discussed in #124823:

This resolves a long held pain point for developers. When using a scroll controller, there is not scroll position until the scrollable widget is built, and almost all methods of notification are only triggered when scrolling happens. Adding these two methods will help developers gain access to the scroll position when it is created. A common workaround for this was using a post frame callback to access controller.position after the first frame, but this is ripe for issues such as having multiple positions attached to the controller, or the scrollable no longer existing after that post frame callback. I think this can also be helpful for folks to debug cases when the scroll controller has multiple positions attached.

Proposal

Add the callbacks to the PageController constructors.