all MGNP frames should include a version

[hawkw]

currently, MGNP lacks any kind of provision for versioning wire frames. this will be an issue if we want to stabilize the wire format in the future.

we should consider adding a version identifier to wire messages now. we could do this either by adding an integer version to the frame header, or by wrapping the entire frame in a

#[derive(Serialize, Deserialize)]
#[non_exhaustive]
pub enum Version {
   V1(Frame),
}

[hawkw]

IMHO the enum is probably the Right Thing, especially because we can then just remove #[non_exhaustive] from most of the other enums. but i think @jamesmunns has opinions on this?