RabbitMQ- Allow Deserialization to complex objects

[PhilKes]

Similiar to #465, it would be great if for the RabbitMQ connector we could automatically deserialize a JSON Payload to a given complex object type.

Something like:

   @Incoming("prices")
    public CompletionStage<Void> consume(Message<Price> price) {
             //...
    }
   
   class Price {
          double amount;
    }

Currently we can only declare the payload of incoming RabbitMQ Message as: byte[], String, JsonObject, JsonArray

[PhilKes]

From what I understand, the IncomingRabbitMQMessage just deserializes the payload according to the contentType in IncomingRabbitMQMessage:

  // Do our best with text and json
  if (HttpHeaderValues.APPLICATION_JSON.toString().equalsIgnoreCase(contentType)) {
      // This could be  JsonArray, JsonObject, String etc. depending on buffer contents
      return body.toJson();
  } else if (HttpHeaderValues.TEXT_PLAIN.toString().equalsIgnoreCase(contentType)) {
      return body.toString();
  }

Could we create a MessageConverter in the rabbitmq module that checks if the Type target is non-primitive, the IncomingRabbitMQMetadata.contentType (or IncomingRabbitMQMessage.contentTypeOverride) is set to JSON and then leverage JsonMapping (if messaging-jsonb or messaging-jackson dependency is present) to achieve this?

I'd be glad to add a PR for this, with some guidance.