✨ Instagram delegate customization (#403)

As we discussed previously #399.
Introduce `InstaAssetPickerBuilder` based on
[insta_assets_picker](https://github.com/LeGoffMael/insta_assets_picker)
which reproduces the Instagram layout.

### This custom delegate shows how to:
- Setup a completely different: layout, select and view logic.
- Use `keepScrollOffset` when set to true to restore scroll and preview.
- Customize `DefaultAssetPickerViewerBuilderDelegate`
  (there is no example for it at present).

It was tested on both iOS and Android.

### Changes needed outside of the example folder
- Fixed an issue with `AssetPickerViewer` when the viewer is being used in nested widgets,
  `initStateAndTicker` would be omitted.

Co-authored-by: Alex Li <[email protected]>

Author: LeGoffMael

CHANGELOG.md

@@ -4,6 +4,13 @@ that can be found in the LICENSE file. -->
 
 # Changelog
 
+## 8.3.1
+
+### New features
+
+- Add `didUpdateViewer` and `initAnimations` in the `AssetPickerViewerBuilderDelegate`. (#403)
+- Add insta_assets_picker as an custom delegate example. (#403)
+
 ## 8.3.0
 
 ### New features

example/lib/customs/custom_picker_page.dart

@@ -6,6 +6,7 @@ import 'package:flutter/material.dart';
 
 import '../constants/custom_pick_method.dart';
 import 'pickers/directory_file_asset_picker.dart';
+import 'pickers/insta_asset_picker.dart';
 import 'pickers/multi_tabs_assets_picker.dart';
 
 class CustomPickersPage extends StatefulWidget {
@@ -51,6 +52,15 @@ class _CustomPickerPageState extends State<CustomPickersPage>
           MaterialPageRoute<void>(builder: (_) => const MultiTabAssetPicker()),
         ),
       ),
+      CustomPickMethod(
+        icon: '📷',
+        name: 'Instagram layout picker',
+        description: 'The picker reproduces Instagram layout with preview and '
+            "scroll animations. It's also published as the package insta_assets_picker.",
+        method: (BuildContext context) => Navigator.of(context).push<void>(
+          MaterialPageRoute<void>(builder: (_) => const InstaAssetPicker()),
+        ),
+      ),
     ];
   }