This implements non-nullable field support in the C++ generator, and fixes some cases where the C++ generation for nullable fields was assuming non-nullable values.

As part of this:

• Factors data class generation out into helper methods.
• Adds isNullable to HostDatatype to make it easier to track nullability across translation from Dart to host datatypes.
• Ports the existing nullable and non-nullable field native unit tests from ObjC to C++, giving native unit test coverage of most of the changes here.
  • Adds a hook in the C++ generator to generate a test friend for a special namespace to facilitate these tests.
• Enables building the non-nullable field file.
• Adjusts the ivar, setter parameter, and getter return types to match nullability expectations.
  • Adds a second convenience setter for nullable types that takes non-nullable values, to facilitate use of literals when constructing objects on the C++ side.

Fixes flutter/flutter#104278

Pre-launch Checklist

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