AMQP: Batch mode option to receive element headers

In preparation for https://github.com/spring-cloud/spring-cloud-stream-binder-rabbit/issues/290

SCSt is not set up to receive a `Message<Message<?>>`.

Similar to the kafka endpoint, add the batched headers in a header.

src/reference/asciidoc/amqp.adoc

@@ -171,6 +171,7 @@ See the https://docs.spring.io/spring-amqp/reference/html/[Spring AMQP Reference
 <27> When the container's `consumerBatchEnabled` is `true`, determines how the adapter presents the batch of messages in the message payload.
 When set to `MESSAGES` (default), the payload is a `List<Message<?>>` where each message has headers mapped from the incoming AMQP `Message` and the payload is the converted `body`.
 When set to `EXTRACT_PAYLOADS`, the payload is a `List<?>` where the elements are converted from the AMQP `Message` body.
+`EXTRACT_PAYLOADS_WITH_HEADERS` is similar to `EXTRACT_PAYLOADS` but, in addition, the headers from each message are mapped from the `MessageProperties` into a `List<Map<String, Object>` at the corresponding index; the header name is `AmqpInboundChannelAdapter.CONSOLIDATED_HEADERS`.
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 [NOTE]

src/reference/asciidoc/whats-new.adoc

@@ -65,3 +65,6 @@ See <<./rmi.adoc#rmi, RMI Support>> for more information.
 
 The outbound endpoints now have a new mechanism for handling publisher confirms and returns.
 See <<./amqp.adoc#alternative-confirms-returns,Alternative Mechanism for Publisher Confirms and Returns>> for more information.
+
+A new `BatchMode.EXTRACT_PAYLOAD_WITH_HEADERS` is supported by the `AmqpInboundChannelAdapter`.
+See <<./amqp.adoc#amqp-inbound-channel-adapter,Inbound Channel Adapter>> for more information.