This PR is to make preparations to make `CardTheme` conform to Flutter's conventions for component themes:
* Added a `CardThemeData` class which defines overrides for the defaults for `Card` properties.
* Added 2 `CardTheme` constructor parameters: `CardThemeData? data` and `Widget? child`. This is now the preferred way to configure a `CardTheme`:
```dart
CardTheme(
data: CardThemeData(color: xxx, elevation: xxx, ...),
child: Card(...)
)
```
These two properties are made nullable to not break existing apps which has customized `ThemeData.cardTheme`.
* Changed the type of theme defaults from `CardTheme` to `CardThemeData`.
TODO:
* Fix internal failures that may have breakages.
* Change the type of `ThemeData.cardTheme` from `CardTheme` to `CardThemeData`. 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
2e7fa83f67