Add loading support to Image (#33369)

This adds two new builders to the `Image` class:

* `frameBuilder`, which allows callers to control the widget
  created by an [Image].
* `loadingBuilder`, which allows callers fine-grained control
  over how to display loading progress of an image to the user.

`FadeInImage` can be simplified by migrating to the new API.
This is done in a follow-on commit.

https://github.com/flutter/flutter/issues/32374

dev/snippets/config/templates/README.md

@@ -86,8 +86,3 @@ follows:
 - [`stateless_widget_scaffold`](stateless_widget_scaffold.tmpl) : Similar to
   `stateless_widget_material`, except that it wraps the stateless widget with a
   Scaffold.
-
-- [`stateful_widget_animation`](stateful_widget_animation.tmpl) : Similar to
-  `stateful_widget`, except that it declares an `AnimationController` instance
-  variable called `_controller`, and it properly initializes the controller
-  in `initState()` and properly disposes of the controller in `dispose()`.

dev/snippets/config/templates/stateful_widget_animation.tmpl

-// Flutter code sample for {{id}}
-
-{{description}}
-
-import 'package:flutter/widgets.dart';
-
-{{code-imports}}
-
-void main() => runApp(new MyApp());
-
-/// This Widget is the main application widget.
-class MyApp extends StatelessWidget {
-  @override
-  Widget build(BuildContext context) {
-    return WidgetsApp(
-      title: 'Flutter Code Sample',
-      home: MyStatefulWidget(),
-      color: const Color(0xffffffff),
-    );
-  }
-}
-
-{{code-preamble}}
-
-/// This is the stateful widget that the main application instantiates.
-class MyStatefulWidget extends StatefulWidget {
-  MyStatefulWidget({Key key}) : super(key: key);
-
-  @override
-  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
-}
-
-// This is the private State class that goes with MyStatefulWidget.
-class _MyStatefulWidgetState extends State<MyStatefulWidget> {
-  AnimationController _controller;
-
-  @override
-  void initState() {
-    super.initState();
-    _controller = AnimationController(
-      vsync: this,
-      duration: const Duration(seconds: 1),
-    );
-  }
-
-  @override
-  void dispose() {
-    _controller.dispose();
-    super.dispose();
-  }
-
-  {{code}}
-}