This PR is to make PageTransitionsTheme conform to Flutter Material's conventions for component themes:

• Added a PageTransitionsThemeData class which defines overrides for the configuration for PageTransitionsTheme.
• Added PageTransitionsTheme constructor parameters: PageTransitionsThemeData? data and Widget? child. This is now the preferred way to configure a PageTransitionsTheme:

 theme: ThemeData(
    pageTransitionsTheme: const PageTransitionsThemeData(
      builders: <TargetPlatform, PageTransitionsBuilder>{
        TargetPlatform.android: xxx,
      },
    ),
  ),

These two properties are made nullable to not break existing apps which has customized ThemeData.pageTransitionsTheme.

• Update PageTransitionsTheme to be an InheritedWidget subclass.
• Add new tests for PageTransitionsTheme and PageTransitionsThemeData, and update the existing PageTransitionsTheme tests.
• This also temporarily changes PageTransitionsThemeData to Object? in ThemeData class, to bypass g3 tests.
• Addresses the "theme normalization" sub-project within ☂️ Material Theme System Updates #91772.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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