protoc-gen-go: Add JSON tags to oneof values and members

[3v1n0]

Oneof values should include json tags to use proper camelCase name and can be safely omitted when we're handling the interface.

[google-cla]

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[puellanivis]

Thank you for your interest into contributing to this project.

Unfortunately this project does not accept GitHub pull requests as the source-of-truth for this project is hosted at https://go.googlesource.com/protobuf. This project page on GitHub is a mirror of that other repository.

If you would like to contribute to this project, please follow the contribution guidelines for instructions on how to send a change. If the change you'd like to make is more substantial or introduces any new features, then it should first be discussed on the issue tracker.

Additionally, be aware that adding JSON tags to oneofs is not compliant with how the JSON mapping standard works. It is generally not recommended to using the standard library encoding/json to convert a protobuf to JSON, precisely because it cannot work in manner that is compliant with the protobuf JSON mapping standards.