Pubsub API has node attribute missing in #event <item> element

Added the optional node attribute to the basic and payload items.

Unfortunately, the donated patch is not sufficient

• it doesn't make the changes in the preferred manner (extending NodeExtension instead of adding a node attribute)

• it doesn't update the test cases (they no longer compile)

• it doesn't fix all the use cases (provider only adds node attribute if there is a payload)

• adds the node attribute to elements where it does not belong (retract item)

This patch cannot be applied as is. The issue itself is still valid though.