[2922] Spec the members command

Include the members command in the spec file of msgq.

src/bin/msgq/msgq.spec

@@ -3,6 +3,18 @@
     "module_name": "Msgq",
     "module_description": "The message queue",
     "config_data": [],
-    "commands": []
+    "commands": [
+      {
+        "command_name": "members",
+        "command_description": "Provide the list of members of a group or of the whole MsgQ if no group is given.",
+        "command_args": [
+          {
+            "item_name": "group",
+            "item_optional": true,
+            "item_type": "string"
+          }
+        ]
+      }
+    ]
   }
 }