Auth rejection message should include optional payload

[connorcarpenter]

When authentication fails during the connection handshake with the Server, it would be helpful to be able to tell the Client why exactly it did fail, e.g. an InvalidCredentials or UserBanned message ect. Which would help the Client take an appropriate action to retry the connection.

The event client-side could look like: Event::Rejection(message: Option<Protocol>) ?

Thank you @uupi for the suggestion from Discord 👍