This PR is to make preparations to make `DialogTheme` conform to Flutter's conventions for component themes:
* Added a `DialogThemeData` class which defines overrides for the defaults for `Dialog` properties.
* Added 2 `DialogTheme` constructor parameters: `DialogThemeData? data` and `Widget? child`. This is now the preferred way to configure a `DialogTheme`:
```
DialogTheme(
data: DialogThemeData(color: xxx, elevation: xxx, ...),
child: Dialog(...)
)
```
These two properties are made nullable to not break existing apps which has customized `ThemeData.dialogTheme`.
* Changed the type of theme defaults from `DialogTheme` to `DialogThemeData`.
TODO:
* Fix internal failures.
* Change the type of `ThemeData.dialogTheme` from `DialogTheme` to `DialogThemeData`. This may cause breaking changes, a migration guide will be created.
Addresses the "theme normalization" sub project within https://github.com/flutter/flutter/issues/91772
* Added support for surfaceTintColor and shadowColor to the Dialog widgets.
* Updated the defaults for Material.shadowColor and Material.surfaceTint to allow turning off the features with a transparent color.
* Added support for shadowColor and surfaceTintColor for Drawer widget.
