Implement experimental REST API endpoints for plugins (#7279)

* Implement experimental REST API endpoints for plugins

* Updates per feedback and rebase

* Update tests

* Further updates

* Update extraction of plugins

* Use OS temp dir for plugins instead of search path

* Fail extraction on paths that attempt to traverse upward

* Update pluginenv ActivePlugins()

config/default.json

@@ -314,6 +314,7 @@
         "RunScheduler": true
     },
     "PluginSettings": {
+        "Enable": false,
         "Plugins": {}
     }
 }