Add config setting to configure plugins directory (#7725)

config/default.json

@@ -335,6 +335,8 @@
     "PluginSettings": {
         "Enable": true,
         "EnableUploads": false,
-        "Plugins": {}
+        "Directory": "./plugins",
+        "Plugins": {},
+        "PluginStates": {}
     }
 }