add support for dual streams in rest:dial (#25)

specs.json

@@ -407,7 +407,8 @@
       "to": "#target",
       "headers": "object",
       "timeout": "number",
-      "amd": "#amd"
+      "amd": "#amd",
+      "dual_streams" : "boolean"
     },
     "required": [
       "call_hook",

test/jambonz-verb-test.js

@@ -313,7 +313,8 @@ test("validate correct verbs", async (t) => {
       "amd": {
         "actionHook": "/answeringMachineDetection",
     
-      }
+      },
+      "dual_streams": true
     }
   ];
   try {