Messages are broadcast to newly connected client before `ServerEvent::ClientConnected` is sent

This is an issue when using bevy_renet.

Assuming a standard plugin setup:

app.add_plugins(RenetServerPlugin);
app.add_plugins(NetcodeServerPlugin);

renet will already broadcast messages to a newly connected client for a single tick before sending ServerEvent::ClientConnected bevy event. This means the game doesn't have the opportunity to react to the client joining first.

The flow is as follows:

### Sometime during Tick 0 ###

- there are already connected clients
- new client connects

### Tick 1 ###

PreUpdate:
- RenetServerPlugin: fn update_system()
- NetcodeServerPlugin: fn update_system()   // new client is added to the list of clients
                                            // ServerEvent::ClientConnected event is queued in RenetServer.events
Update:
- app: game logic queues new messages

PostUpdate:
- transport: fn send_packets()  // messages are sent to everyone including new client

### Tick 2 ###

PreUpdate:
- RenetServerPlugin: fn update_system()     // sends queued ServerEvent::ClientConnected event to bevy
- NetcodeServerPlugin: fn update_system()

Update:
- app: game logic queues new messages   // also reacts to ClientConnected, queues client setup messages

PostUpdate:
- transport: fn send_packets()  // new client receives setup messages, but after regular messages from Tick 1

The issue comes from the fact that in NetcodeServerPlugin, the update method is set to run after the one from RenetServerPlugin:

# transport.rs
impl Plugin for NetcodeServerPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<NetcodeTransportError>();

        app.add_systems(
            PreUpdate,
            Self::update_system
                .run_if(resource_exists::<NetcodeServerTransport>())
                .run_if(resource_exists::<RenetServer>())
                .after(RenetServerPlugin::update_system),
        );

// ...

So during the first tick, RenetServerPlugin does not have any events to send.

Expected behaviour

After a new client connects, ServerEvent::ClientConnected should be sent before send_packets() is called. This would allow the game to react to the new arrival before sending them any messages.

lucaspoffo / renet

Messages are broadcast to newly connected client before `ServerEvent::ClientConnected` is sent #106

Expected behaviour