Instead of generating a default constructor for C++ data classes that will leave non-nullable POD fields uninitialized (and other non-nullable fields default-initialized, which doesn't match what we do for other languages), this creates either one or two initializers:

• One that has parameters for all non-nullable fields.
• If there are any nullable fields, one that has parameters for all fields as a connivence constructor. (With the caveat that passing values for nullable values is not super convenient since they are pointer arguments.)

Other changes that followed from this:

• The deserialization function is now a factory rather than a constructor; this was necessary to avoid a collision in the case of a data class whose only non-nullable field is an EncodableList.
• Deserialization now uses the constructor for non-nullable arguments. (Technically we could have kept a private default constructor and not changed it, but doing it this way makes it easier to reason about not accidentally leaving anything uninitialized).
  • While touching that code, I simplified the handling on non-nullable values to unconditionally read (vs silently ignoring null or wrong-type values), and adjusted the nullable handling to check for null and then unconditionally read (vs silently treating wrong-type values as null), in line with changes in other languages to be stricter about type casting.
• ErrorOr's constructors had to be adjusted slightly, as they had a hidden default initialization of T that became an error once the default constructor for data classes went away.

Fixes flutter/flutter#104653
Fixes flutter/flutter#104286

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