Add support for spring-amqp

[lyca]

I know that one could chain the org.springframework.amqp.support.converter.MessagingMessageConverter with the io.cloudevents.spring.messaging.CloudEventMessageConverter, but a more direct approach also honoring the AMQP Protocol Binding for CloudEvents is more desirable.

[lyca]

I just released a package with the content of the pull request as a third party package. Maybe someone else finds it useful :smile: https://central.sonatype.com/artifact/de.lyca.cloudevents/cloudevents-spring-amqp/overview