pubsub: Add a PubSubPayload trait.

src/pubsub/mod.rs

@@ -45,3 +45,6 @@ generate_attribute!(
         Unconfigured => "unconfigured",
     }, Default = None
 );
+
+/// This trait should be implemented on any element which can be included as a PubSub payload.
+pub trait PubSubPayload: crate::TryFrom<crate::Element> + Into<crate::Element> {}