Missing ce_time header for CloudEvent feature

[vietk]

When providing the cloud event metadata using the CloudEventContextHandler#setOutgoingContext, we are forced to supply the value for the time of the creation of the event.

   CloudEvent cloudEvent = cloudEventContextHandler.contextBuilder()
        .withSubject("mySubject")
        .withSource(URI.create("mysource")
        .withTime(OffsetDateTime.now())
        .build();

In order to reduce boiler plate code, the extension could creates this metadata when the message is serialized a bit like what reactive messaging is doing with cloud-events-insert-timestamp (default is true)