Add Producer events

[nathanjrobertson]

Currently, Consumers have events which allow users of the bundle to add custom code. This PR adds this support to Producers as well. Really useful for project specific logging, message format validation, etc.

The only real difference between the Consumer and Producer events is the addition of the routingKey field in the Events, as $producer->routingKey is only set in the case where it is explicitly set, and not in the case where defaultRoutingKey is used. This PR currently works around this by providing that extra field, but it might be better to change Producer.php (line 67) from:

$real_routingKey = $routingKey !== null ? $routingKey : $this->defaultRoutingKey;

to

if ($routingKey === null) {
    $routingKey = $this->defaultRoutingKey;
}

and then removing the extra routingKey field from the Events. Happy to take feedback on which way you'd prefer this one go.

My use case for this support is message validation prior to publishing.