This PR adds an another example for SliverAppBar, showing the use of stretch and onStretchTrigger

https://user-images.githubusercontent.com/68919043/235420973-2bfb9871-9e05-4d87-9538-941d43178c76.mp4

Fixes #125651 

### Adds sample code for SliverAppBar [stretch, onStretchTrigger]

This PR adds an another simple and easily understandable example code for SliverAppBar.