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