footer property
footer indicator display after content
If reverse is true,header displace at the top of content. If reverse is false,header displace at the bottom of content. if scrollDirection = Axis.horizontal,it will display at left or right
from 1.5.2,it has been change LoadIndicator to Widget,but remember only pass sliver widget,
Implementation
// if you pass not a sliver,it will throw error
final Widget? footer;