Add Channel's status.deadLetterSinkURI behavior to spec

The [Channel lifecycle specification](https://github.com/knative/specs/blob/main/specs/eventing/control-plane.md#channel-lifecycle) doesn't include the desired behavior of the status.deadLetterSinkURI (as the Trigger does).
It should be added to it's spec (as optional at least)

Open specs/eventing/control-plane.md and read the Channel lifecycle section, then compare how status.deadLetterSinkURI is described for Trigger. Add the corresponding optional Channel behavior; the section is done when it documents this field consistently with the Trigger specification.

Written by the indexing model from the issue text.