have an option to not generate the pbjson.dart files

I believe these aren't generally needed for most proto / grpc use cases. Perhaps it should be an option to opt-in to their generation? But at least having an option to not generate them would be good; currently people handle this by generating all files, and then deleting the unneeded ones after.

Start by locating the Dart protobuf or gRPC generation entry point that produces pbjson.dart files, then inspect how generation options are defined and tested. The work is done when users can opt out of pbjson.dart generation while other generated files remain available, with coverage for the new option.

Written by the indexing model from the issue text.