[material_ui] Add contrastLevel for M3 ColorScheme

[QuncCccccc]

This PR is to port #11801

Original PR Description

Fixes flutter/flutter#184951

This PR is to add medium- and high-contrast M3 ColorScheme baselines. I also added a double? contrastLevel to ThemeData.

By default, ThemeData() shows standard M3 ColorScheme. To choose medium/high contrast, use

ThemeData(
  // brightness: Brightness.dark shows dark version of the baselines.
  contrastLevel: ContrastLevel.medium, // ContrastLevel.high
)

This change will not cause breaking changes because the standard-contrast m3 color scheme baseline stays the same.

TODO:
Add an example or update an existing example to reflect the new feature.