Implement alert_endpoints and http_lint plugin extensions

Alert endpoints can now be loaded via custom plugins.
It's now possible to extend the http_lint parameters with custom parameters and validators.

scripts/plugins/syslog_alert_endpoint/plugin.lua

@@ -0,0 +1,17 @@
+--
+-- (C) 2019 - 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 = "Syslog Alert Endpoint",
+  description = "Provides alerts notifications to Syslog",
+  author = "ntop",
+  version = 1,
+  dependencies = {},
+}