Reworks plugin loading and structure

Implements #4358

scripts/plugins/endpoints/sqlite_alert_endpoint/manifest.lua

@@ -0,0 +1,16 @@
+--
+-- (C) 2019-20 - ntop.org
+--
+
+-- This file contains the plugin metadata information.
+
+-- NOTE: Plugins are loaded at startup, any change to this plugin
+-- can be applied by restarting ntopng or by manually reloading the
+-- plugins from http://127.0.0.1:3000/lua/plugins_overview.lua .
+
+return {
+  title = "SQLite Alert Endpoint",
+  description = "Handles SQLite alert notifications for the UI",
+  author = "ntop",
+  dependencies = {},
+}