Testing Issue: breaking changes related to grpc generator

[shollyman]

As part of showcase testing in this generator, CI tests effectively compare a recent release of the showcase generated code to one that's output by the generator using apidiff.

The grpc plugin (https://pkg.go.dev/google.golang.org/grpc/cmd/protoc-gen-go-grpc) has a new option (use_generic_streams_experimental) that recently defaulted to true. Because our showcase testing grabs the plugin latest version, we were picking up that new change.

To resolve our immediate issue, we need to add the experimental flag to our build process to disable the newly enabled behavior, and verify what further actions are needed.

[quartzmo]

Great diagnosis, thank you!