Consider making generated Protobuf types immutable

[dmdashenkov]

In Java, Protobuf types are generated immutable. This approach has a lot of benefits.

We should consider tweaking the Dart code generation to make Dart Protobuf types immutable as well.

[dmdashenkov]

There's a lib for that: https://pub.dev/packages/freezed.

[alexander-yevsyukov]

Also this project (https://github.com/comigor/artemis) can probably give us some code-generation hints.

[armiol]

See the related issues:

• API for creating read-only protobufs google/protobuf.dart#53 (to make the generated messages freezed)
• Submessages are read only by default google/protobuf.dart#305 (to make the sub-messages mutable :))