mgrpc: baseline protobuf example

Set the stage for the example server by including a service definition,
generated code and the protobuild definition file used to generate them.
From here, we can work up to a generated mgrpc server.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/protoc-gen-gogomgrpc/main.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"github.com/gogo/protobuf/protoc-gen-gogo/descriptor"
+	"github.com/gogo/protobuf/vanity"
+	"github.com/gogo/protobuf/vanity/command"
+	_ "github.com/stevvooe/mgrpc"
+)
+
+func main() {
+	req := command.Read()
+	files := req.GetProtoFile()
+	files = vanity.FilterFiles(files, vanity.NotGoogleProtobufDescriptorProto)
+	for _, opt := range []func(*descriptor.FileDescriptorProto){
+		vanity.TurnOffGoGettersAll,
+		vanity.TurnOffGoStringerAll,
+		vanity.TurnOnMarshalerAll,
+		vanity.TurnOnStringerAll,
+		vanity.TurnOnUnmarshalerAll,
+		vanity.TurnOnSizerAll,
+	} {
+		vanity.ForEachFile(files, opt)
+	}
+
+	resp := command.Generate(req)
+	command.Write(resp)
+}