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/example/alert_endpoints/example.lua

@@ -0,0 +1,89 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+require "lua_utils"
+local json = require "dkjson"
+
+local script = {}
+
+-- How often this script will be called (in seconds)
+script.EXPORT_FREQUENCY = 5
+
+-- The minimum severity for an alert in order to be exported by this
+-- endpoint
+script.DEFAULT_SEVERITY = "warning"
+
+-- This determines the invocation priority of this endpoint.
+-- Higher priority endpoints are invoked first for the alert export.
+script.prio = 500
+
+-- ##############################################
+
+-- Each endpoint has a dedicated redis queue for the alerts. This function
+-- is called every EXPORT_FREQUENCY seconds and should check the alerts
+-- queue for alerts and possibly export them.
+
+-- @brief Process the pending alerts notifications from the queue
+-- @params queue the redis queue name
+-- @return {success = true} on success,
+-- {success = false, error_message = "Some error description here"} on failure
+function script.dequeueAlerts(queue)
+  while true do
+    local json_alert = ntop.lpopCache(queue)
+
+    if not json_alert then
+      break
+    end
+
+    local alert = json.decode(json_alert)
+
+    -- Print the alert on the console.
+    tprint(alert)
+
+    -- Can filter the alerts based on some criteria
+    if (alert.action == "engage") then
+      if (alertTypeRaw(alert.type) == "alert_threshold_cross") and
+            (alert.alert_key == "min_active_local_hosts") then
+
+        -- Export the alert, e.g. by running a custom bash script
+        --os.execute("/tmp/my_script.sh")
+      end
+    end
+  end
+
+  return {success=true}
+end
+
+-- ##############################################
+
+-- @brief Called when the "Alert Endpoints" form is submitted.
+-- @note This API could be subject to change!
+function email.handlePost()
+  --tprint(_POST)
+end
+
+-- ##############################################
+
+-- @brief Adds some custom preferences to the "Alert Endpoints" page
+-- @brief alert_endpoints a reference to the scripts/lua/modules/alert_endpoints_utils.lua module
+-- @brief subpage_active the lua table representing the currently active page
+-- @brief showElements right now is always true
+-- @note This API could be subject to change!
+function script.printPrefs(alert_endpoints, subpage_active, showElements)
+  local elementToSwitch = {}
+
+  print('<thead class="thead-light"><tr><th colspan="2" class="info">'..i18n("example.example_notification")..'</th></tr></thead>')
+
+  prefsToggleButton(subpage_active, {
+    field = "toggle_example_notification",
+    pref = alert_endpoints.getAlertNotificationModuleEnableKey("example", true),
+    default = "0",
+    disabled = showElements==false,
+    to_switch = elementToSwitch,
+  })
+end
+
+-- ##############################################
+
+return script