#623 Adds custom gogoproto.oneof_moretags option to OneofOptions

[raphaelzoellner]

This PR allows to specify the gogoproto.oneof_moretags option in oneof fields. This allows passing additional tags to the generated interface fields of go structs for oneof fields.

This feature refers to the issue described here.

syntax = "proto3";
  
import "gogo/protobuf/gogoproto/gogo.proto";

message T {
  oneof v {
    option (gogoproto.oneof_moretags) = "tag:\"val\"";
    ...
  }
}