Add a flag to whether to generate pb file

[wuqinqiang]

Is your feature request related to a problem? Please describe.

Currently, PB files do not support external imports, which makes it inconvenient to directly apply certain plugins as they require additional processing. For instance, basic plugins like protoc-gen-validate can not be used directly.

The ProtocCmd assembled by the GenPb function within goctl is somewhat insufficient. For my purposes, I only want to generate the framework's core code. As for managing and generating pb files, I prefer using buf.

demo

Therefore, I'd like to add a flag that allows skipping the generation of GenPb and only generates the core framework code.

Describe the solution you'd like
Add a flag: --pb

# skip generate GenPb 
goctl rpc protoc proto/demo2.proto --pb=false  ...........

Describe alternatives you've considered

Additional context

[wuqinqiang]

@kevwan Will this be open -_-?