Implement getter for protocol constants

Implements ##3945

scripts/lua/rest/v1/get/l4/protocol/consts.lua

@@ -0,0 +1,39 @@
+--
+-- (C) 2013-20 - ntop.org
+--
+
+local dirs = ntop.getDirs()
+package.path = dirs.installdir .. "/scripts/lua/modules/?.lua;" .. package.path
+
+require "lua_utils"
+local json = require "dkjson"
+local rest_utils = require "rest_utils"
+
+--
+-- Read all the  L4 protocols
+-- Example: curl -u admin:admin http://localhost:3000/lua/rest/v1/get/l4/protocol/consts.lua
+--
+-- NOTE: in case of invalid login, no error is returned but redirected to login
+--
+
+sendHTTPHeader('application/json')
+
+local rc = rest_utils.consts_ok
+local res = {}
+
+for _, l4_key in pairs(l4_keys) do
+   -- l4_keys example structure
+   -- table
+   -- 1 table
+   -- 1.1 string IP
+   -- 1.2 string ip
+   -- 1.3 number 0
+   -- 2 table
+   -- 2.1 string ICMP
+   -- 2.2 string icmp
+   -- 2.3 number 1
+   res[l4_key[1]] = {proto_id = l4_key[3]}
+end
+
+print(rest_utils.rc(rc, res))
+