make it easier to configure dispatcher for Camel ProducerChild, #23099

* and use parent dispatcher as default, better than default-dispatcher

(cherry picked from commit c911c06b9e60b88874cf9b608c64aa5a125e9794)

akka-camel/src/main/resources/reference.conf

@@ -32,6 +32,15 @@ akka {
       # The duration of time to await activation of an endpoint.
       activation-timeout = 10s
     }
+    
+    producer {
+      # The id of the dispatcher to use for producer child actors, i.e. the actor that
+      # interacts with the Camel endpoint. Some endpoints may be blocking and then it
+      # can be good to define a dedicated dispatcher.
+      # If not defined the producer child actor is using the same dispatcher as the 
+      # parent producer actor.
+      use-dispatcher = ""
+    }
 
     #Scheme to FQCN mappings for CamelMessage body conversions
     conversions {

akka-camel/src/main/scala/akka/camel/Camel.scala

@@ -87,6 +87,8 @@ class CamelSettings private[camel] (config: Config, dynamicAccess: DynamicAccess
    */
   final val StreamingCache: Boolean = config.getBoolean("akka.camel.streamingCache")
 
+  final val ProducerChildDispatcher: String = config.getString("akka.camel.producer.use-dispatcher")
+
   final val Conversions: (String, RouteDefinition) ⇒ RouteDefinition = {
     val specifiedConversions = {
       import scala.collection.JavaConverters.asScalaSetConverter

akka-camel/src/main/scala/akka/camel/Producer.scala

@@ -59,7 +59,11 @@ trait ProducerSupport extends Actor with CamelSupport {
   protected def produce: Receive = {
     case CamelProducerObjects(endpoint, processor) ⇒
       if (producerChild.isEmpty) {
-        producerChild = Some(context.actorOf(Props(new ProducerChild(endpoint, processor))))
+        val disp = camel.settings.ProducerChildDispatcher match {
+          case "" ⇒ context.props.dispatcher
+          case d  ⇒ d
+        }
+        producerChild = Some(context.actorOf(Props(new ProducerChild(endpoint, processor)).withDispatcher(disp)))
         messages = {
           for (
             child ← producerChild;