[Feature Request] Argments to depress `pbserver.dart` generation

# Background

Sometimes we just need the protobuf encode/decode feature, especially for embedded environtment

# Expect

```sh
protoc --dart_out=lib/proto -Iproto proto/*.proto --no-server
```

Then we got `*.pb.dart`, `*.pbenum.dart` and `*.pbjson.dart` only

Start by tracing the protoc Dart generator path responsible for producing pbserver.dart files, using the requested --no-server invocation as the expected interface. Check how generated outputs are selected and whether generator tests cover server files. Done means the option is accepted and produces only *.pb.dart, *.pbenum.dart, and *.pbjson.dart files.

Written by the indexing model from the issue text.