This fixes #71222

* Implement operator = and hashcode for CupertinoThemeData and CupertinoTextThemeData which CupertinoThemeData depends on.
* Add basic tests for CupertinoThemeData equality and hashcode.

I also manually tested the case mentioned in #71222, e.g. equality of ThemeData(cupertinoOverrideTheme: someTheme)
Co-authored-by: Jacob Simionato <jsimionato@google.com>