feat：SearchAppBar增加最大长度限制属性

lib/widget/search_app_bar.dart

@@ -24,6 +24,7 @@ class SearchAppBar extends StatefulWidget implements PreferredSizeWidget {
   final TextEditingController? textEditingController;
   final double? leadingWidth;
   final double? searchBgRadius; // 搜索背景圆角
+  final int? maxLength; // 最大个数
 
   const SearchAppBar({
     Key? key,
@@ -48,6 +49,7 @@ class SearchAppBar extends StatefulWidget implements PreferredSizeWidget {
     this.textEditingController,
     this.leadingWidth,
     this.searchBgRadius,
+    this.maxLength,
   }) : super(key: key);
 
   @override
@@ -110,6 +112,7 @@ class _SearchAppBarState extends State<SearchAppBar> {
                     Expanded(
                       child: TextField(
                         maxLines: 1,
+                        maxLength: widget.maxLength,
                         focusNode: _searchFocusNode,
                         style: TextStyle(
                             fontSize: 14,