Description
Currently there is no option to configure message flags when publishing via Amqp Transport. Flag AMQP_NOPARAM is hardcoded into the publishing function: https://github.com/symfony/messenger/blob/v4.1.2/Transport/AmqpExt/Connection.php#L101.
I think it should be configurable by the developer as there is no way to publish e.g. persistent messages. Once this Feature Request is accepted I am happy to provide PR myself.

Example
I imagine that it would look like this in the code:

$message = new MyMessage("Hello world");
$bus->dispatch((new Envelope($message))->withFlags(AMQP_DURABLE));