feat: Deprecate the `Actor.email` field and add `username` to replace it
feat: Annotate `Comment.plain_text_body` as deprecated (it was already deprecated, but now we're adding the annotation for it)
PiperOrigin-RevId: 758717798
Note: For the time being, you can't upload or download attachments. We will look into supporting this feature in the future.
PiperOrigin-RevId: 527954020
