signalr: adding the missing Trace value (Azure#15455)

Fixes Azure#14923

Author: tombuildsstuff

specification/signalr/resource-manager/Microsoft.SignalRService/stable/2020-05-01/signalr.json

@@ -1418,7 +1418,8 @@
       "enum": [
         "ClientConnection",
         "ServerConnection",
-        "RESTAPI"
+        "RESTAPI",
+        "Trace"
       ],
       "type": "string",
       "x-ms-enum": {