Allow adding a stick subheader component in MODAL list mode

[lucasbasquerotto]

Currently, I can add a subheader with:

<DropDownPicker
    listMode="MODAL"
    flatListProps={{ ListHeaderComponent: () => <MyComponent />}
    ...
/>

I want to be able to add a stick subheader. It will show any error that may happen when the user select an option in the stick header, even when the user had scrolled, otherwise (the way it is today) the user may not see the error and not understand what is happening.

The flatListProps has an option StickyHeaderComponent, but it does nothing (maybe it's overwritten by the DropDownPicker component own header; if this is the case, it could just add this property, when defined, as a child).