feat: add onhold music to dial verb

specs.json

@@ -213,7 +213,8 @@
       "timeout": "number",
       "proxy": "string",
       "transcribe": "#transcribe",
-      "amd": "#amd"
+      "amd": "#amd",
+      "onHoldMusic": "string"
     },
     "required": [
       "target"

test/jambonz-verb-test.js

@@ -68,6 +68,7 @@ test("validate correct verbs", async (t) => {
       "callerName": "Tom",
       "answerOnBridge": true,
       "dtmfCapture": ["*2", "*3"],
+      "onHoldMusic": "http://server.com/hold",
       "dtmfHook": {
         "url": "/dtmf",
         "method": "GET"