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

[Sunbreak]

Background

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

Expect

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

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

[osa1]

Removing pbserver files was also discussed in #487 (comment) and the next comment.