Ability to prevent field rewriting option for the "grpc-tools"

### Is your feature request related to a problem? Please describe.
For the generated models by the `grpc-tools` javascript will contain the `toObject` method.
We noticed that by default it has the following behavior:
- `my_prop` will be converted to `myProp`
- `repeated Users users = 1;` will be converted to `usersList`

### Describe the solution you'd like
Please correct me if we missed something, but it would be great to introduce such kind of option like `--keepFieldsNames` to save the original names from proto.