Expose scroll position and layout measurements on CategoryOverflowCalendarDayView

[maksym-tielnyi-solid]

Description

This PR exposes some internals of the CategoryOverflowCalendarDayView widget, allowing users to build layouts that depend on its current measurements and scroll positions. For example, this can be helpful to build custom overlays and out-of-view indicators.

• Added the onLayoutMetrics callback parameter to the CategoryOverflowCalendarDayView constructor, reporting the body content size and the body viewport measurements of the calendar view after each layout pass.
• Added the horizontalScrollControllerGroup and verticalScrollControllerGroup parameters to the CategoryOverflowCalendarDayView constructor, letting users read, observe, and drive the view's scroll position.
• Re-exported LinkedScrollControllerGroup from the package so these parameters can be used without a direct dependency on linked_scroll_controller.
• Disposed of the internally-created scroll controllers inside _CategoryOverflowCalendarDayViewState.