Add event reply header to spec (#4560)

* Add event reply header to spec

* Refactor wording

* Fix nits

* Use K-Eventing-Http-Attr instead

* Replace all the MUST with SHOULD

* Use prefer: callable as a hint to the sink for event reply support

* Use single prefer header instead of an array

* Use reply instead of callable

docs/spec/data-plane.md

@@ -91,6 +91,21 @@ delivered.
 CloudEvents received by Sink MAY have
 [Distributed Tracing Extension Attribute](https://github.com/cloudevents/spec/blob/v1.0/extensions/distributed-tracing.md).
 
+### Event reply contract
+
+An event sender supporting event replies SHOULD include a `Prefer: reply` header
+in delivery requests to indicate to the sink that event reply is supported. An event
+sender MAY ignore an event reply in the delivery response if the `Prefer: reply`
+header was not included in the delivery request.
+
+An example is that a Broker supporting event reply sends events with an additional header
+`Prefer: reply` so that the sink connected to the Broker knows event replies will
+be accepted. While a source sends events without the header, in which case the sink may
+assume that any event reply will be dropped without error or retry attempt. If a sink
+wishes to ensure the reply events will be delivered, it can check for the existence of
+the `Prefer: reply` header in the delivery request and respond with an error code
+if the header is not present.
+
 ### Data plane contract for Sources
 
 See [Source Delivery specification](../spec/sources.md#source-event-delivery)