Use case

Because 'background' can only pass one fixed component and cannot display different component contents according to 'FlexibleSpaceBar' stretch, we need a way to build a 'background' according to the stretch state.

Proposal

add backgroundBuilder, expose 'FlexibleSpacebarSettings' and' BoxConstraints' to users to build components more flexibly according to stretch state.

Widget (BuildContext context, FlexibleSpaceBarSettings settings, BoxConstraints constraints) backgroundBuilder 

`Background 'and' backgroundBuilder' can't be used at the same time. Retaining 'background' ensures forward compatibility.