Opt in to NetMsg trait

[MitchellMarinoDev]

Right now the NetMsg trait has a generic implementation for all types that implement Downcast + Send + Sync + Debug. Instead, it could be better to switch this to a derive where you have to opt-in to your type being a NetMsg.

This would allow us to have a derive macro which specifies the Reliability requirements. It would make more sense to specify those requirements next to the message definition, rather than when you register it.