feat：SearchAppBar设置阴影

lib/widget/search_app_bar.dart

@@ -26,6 +26,7 @@ class SearchAppBar extends StatefulWidget implements PreferredSizeWidget {
   final double? leadingWidth;
   final double? searchBgRadius; // 搜索背景圆角
   final int? maxLength; // 最大个数
+  final double? elevation; // 阴影
 
   const SearchAppBar({
     Key? key,
@@ -51,6 +52,7 @@ class SearchAppBar extends StatefulWidget implements PreferredSizeWidget {
     this.leadingWidth,
     this.searchBgRadius,
     this.maxLength,
+    this.elevation,
   }) : super(key: key);
 
   @override
@@ -85,6 +87,7 @@ class _SearchAppBarState extends State<SearchAppBar> {
       leading: widget.leading,
       leadingWidth: widget.leadingWidth,
       automaticallyImplyLeading: widget.automaticallyImplyLeading,
+      elevation: widget.elevation,
       title: Row(
         children: <Widget>[
           Expanded(