C#

- Add GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE compiler variable to enable compilation without reflection (C# 7.3, or pre-roslyn. May work better for il2cpp pipelines)

- "ControlEvent" has a flaw in design coupling subprotocols in the main protocol. We should seek consistency.

- Naming in messages is utterly broken, many messages are named "Report" where those are Responses to requests. "Report" is a proactive naming, for notifications. Examples:
- SendScreenshot {id, data} the name should be ScreenshotResponse { requestId, data }
- ReportBuilderCameraTarget{id,vec3} should be BuilderRaycastResponse{id,vec3}