Adopt CloudEvents-based message format for event bus

[arnoweiss]

Context

For a while, there was development on a fork of eclipse-fa3st/fa3st-service in the factory-x-contributions org. The implementation follows the format that was specified in the async-aas spec that's currently in discussion for adoption in the IDTA's spec group. It nests AAS objects in a CloudEvents envelope.

Feature Set

Compared to the events currently implemented in FAAAST, the proposal covers a subset only. It omits the transfer of the oldValue as part of the event and even (as per spec) allows to omit the data entirely. Other than that, its events approximately represent reports on synchronous CUD (no R) operations.

Interface Changes

To the admin, the implementation introduces a new configuration variable that represents a callback url via which a subscriber can synchronously fetch the data.

To the subscriber, the payload structure and event metadata changes.

Context

Despite being developed on a fork of the eclipse repo, it would be possible to rebase the content into this repo's git tree and contribute it via PR on short notice.

[carlos-schmidt]

After internal discussions, we have decided that we will adopt the CloudEvents message bus into the upstream project. I will look at the technical side of getting this feature upstream and will circle back to this issue once the move has finished.

[mjacoby]

@arnoweiss The CloudEvents integration is now available on the main branch. To use it you have to configure FA³ST Service to use the CloudEvents MessageBus implementation https://faaast-service.readthedocs.io/en/latest/interfaces/message-bus.html#cloudevents