Add a support for oneof

[goj]

https://developers.google.com/protocol-buffers/docs/proto#oneof

Generating enum and being able to switch on which field is set (like in Java proto library) would be especially useful.

[wienczny]

👍
Currently oneof seems to be somewhat supported. They are generated as regular fields by dart-protoc-plugin. Support for selecting and getting the current field would be nice. But I think this is an dart-protoc-plugin issue.

[gamebox]

I don't think this would be terribly difficult to implement, and not having it really dampens the usefulness of the plugin for the use cases in which I typically want to use protobuf.

Unless this assigned internally, I wouldn't mind submitting a PR as I'm hoping to use this for a Flutter side-project.

[szakarias]

PRs are always welcome, but I am also starting to look at this, so it should be supported soon.

[szakarias]

Fixed by #166 .