PLT-7407: Back-end plugins (#7409)

* tie back-end plugins together

* fix comment typo

* add tests and a bit of polish

* tests and polish

* add test, don't let backend executable paths escape the plugin directory

api4/plugin_test.go

@@ -110,5 +110,5 @@ func TestPlugin(t *testing.T) {
 	_, resp = th.SystemAdminClient.RemovePlugin("bad.id")
 	CheckNotFoundStatus(t, resp)
 
-	th.App.Srv.PluginEnv = nil
+	th.App.PluginEnv = nil
 }