[5.4] Model binding in broadcasting channel definitions

[themsaid]

This PR allows model binding on channel authorization:

Broadcast::channel('order.{order}', function ($user, App\Order $order) {
    return $user->id === $order->user_id;
});

You can still pass normal values:

Broadcast::channel('order.{order}, function ($user, $orderId) {
    return $user->id === Order::findOrNew($orderId)->user_id;
});

If model wasn't found its value will be null:

Broadcast::channel('order.{order}', function ($user, App\Order $order) {
    // $order will be null in case we couldn't find the model
});

Unlike models, the key names here can be anything:

Broadcast::channel('order.{order_reference_number}', function ($user, App\Order $order) {
    return $user->id === $order->user_id;
});

Note that we use Model::getRouteKeyName to get the key name instead of having a separate method for the broadcaster.

[morloderex]

This looks nice :)

[themsaid]

Because the first parameter is always the logged in user instance.