Make SwiftMailerConsumer optional

I'm using SonataNotificationBundle in an application where SwiftmailerBundle is disabled, since it will send no emails at all. But SonataNotificationBundle is forcing me to enable the bundle for service sonata.notification.consumer.swift_mailer, which is registered automatically and has a dependency on the mailer service.

Would be nice to enable/disable the SwiftMailerConsumer on demand.