Previously, we used the full reference to the `OutEvent` of the `ConnectionHandler` in all implementations of `NetworkBehaviour`. Not only is this very verbose, it is also more brittle to changes. With the current implementation plan for #2824, we will be removing the `IntoConnectionHandler` abstraction. Using a type-alias to refer to the `OutEvent` makes the migration much easier.
Storing `NetworkBehaviourAction`s within the behaviour is more flexible than only storing `OutEvent`s. Additionally, I find expression-oriented code easier to reason about because it typically doesn't contain side-effects.
