This PR is to make preparations to make `TabBarTheme` conform to Flutter's conventions for component themes:
* Added a `TabBarThemeData` class which defines overrides for the defaults for `TabBar` properties.
* Added 2 `TabBarTheme` constructor parameters: `TabBarThemeData? data` and `Widget? child`. This is now the preferred way to configure a `TabBarTheme`:
```
TabBarTheme(
data: TabBarThemeData(labelColor: xxx, indicatorColor: xxx, ...),
child: TabBar(...)
)
```
These two properties are made nullable to not break existing apps which has customized `ThemeData.tabBarTheme`.
* Changed the type of component theme defaults from `TabBarTheme` to `TabBarThemeData`.
TODO:
* Fix internal failures.
* Change the type of `ThemeData.tabBarTheme` from `TabBarTheme` to `TabBarThemeData`. 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
f310625bc6