Add Tags to protobuf/reflect

[joesonw]

Is your feature request related to a problem? Please describe.
Allow attach tags to protoobuf/reflect and have them being injected into final *.pb.go can easily allow inject custom tags.

Describe the solution you'd like

• add tags to FieldDescriptor
• populate tags in generated .pb.go in protoprotocolbuffers/protobuf-go (specifically here)

This allows custom plugins to modify FieldDescriptor tags and wrap protoc-gen-go's GenerateFile thus enables injecting custom tags.

Describe alternatives you've considered
All existing methods of injecting tags end up replacing generated go files.

Additional context
#52 it has being long requested.

[dsnet]

Closing as duplicate of #52. There is no need to keep duplicate issues around.