Move Event version to metadata instead of having hard coded version on event type

[matzew]

Feature request

I see that there are a couple of Tekton events, that have their version hard coded into their type/fqn. See:

triggers/pkg/reconciler/events/event.go

Lines 25 to 35 in 176dc0f

	// TriggerProcessingStartedV1 is sent for Sink Triggers when a trigger is started
	TriggerProcessingStartedV1 = "dev.tekton.event.triggers.started.v1"
	// TriggerProcessingSuccessfulV1 is sent for Sink Triggers when a trigger is successful
	TriggerProcessingSuccessfulV1 = "dev.tekton.event.triggers.successful.v1"
	// TriggerProcessingFailedEventV1 is sent for Sink Triggers when we fail to process trigger
	TriggerProcessingFailedV1 = "dev.tekton.event.triggers.failed.v1"
	// TriggerProcessingDoneV1 is sent for Sink Triggers when we are done
	// with eventlistener handler
	TriggerProcessingDoneV1 = "dev.tekton.event.triggers.done.v1"
	// EventAccepted is sent as response for CloudEvent compliant providers
	EventAccepted = "dev.tekton.event.triggers.accepted.v1"

I think if the event type was more generic, and the version would be stored on among the metadata of an event (instead of the event type itself) we have a cleaner design. In CloudEvent this could be done via an extension.

The same approach is now also take by the CNCF xregistry (spin-off from CloudEvents), event/message definitions can be grouped, and each definition/message (aka event) can have its own version as metadata/property:
https://github.com/xregistry/spec/blob/main/message/spec.md

[dibyom]

/cc @afrittoli @khrm thoughts?